Of course, America's cultural and historical dominance of the 20th century may be a factor.
Our currently designated cultural sites, though, aren't even American. Two are earthworks (Poverty Point and Cahokia) and three are from Southwest cultures (Mesa Verde, Taos Pueblo, and Chaco Canyon). Besides these five indigenous selections, there are two from the Spanish (La Fortaleza in Puerto Rico and San Antonio Missions) and two from the British colonial era (Independence Hall and Monticello). The only UNESCO property on the list that post-dates 1776 is the Statue of Liberty - a gift from the French. A single mixed property on the list (considered both natural and cultural) is Papahanamoukoukea. This site, of sacred value to native Hawaiians I believe as the origins of creation, has no actual structure, or monument, just cultural religious significance.
Back in 2014 I mentioned which American sites were being considered for inclusion, on the 'tentative' list, and the list hasn't changed much (they added the San Antonio missions, expanded Monticello). Below is that updated enumeration, only here it's only the cultural properties under consideration (the natural properties still on the docket are the Okefenokee, White Sands, Petrified Forest National Park, and Fagatele Bay. Except White Sands I'm down.)
1. Civil Rights Movement Sites (Dexter Avenue King Memorial Baptist Church, Bethel Baptist Church, 16th Street Baptist Church)
Sure. I think the US Civil Rights movement is of global significance. Heck, Robben Island in South Africa is a UNESCO site, so why not our buildings?
2. Dayton Aviation Sites (Huffman Prairie Flying Field, Wright Cycle Company and Wright & Wright Printing, Wright Hall, Hawthorn Hill)
Sure. We invented flight, it happened at a certain place, let’s commemorate that place. Odd that it's not Kittyhawk, but whatever.
3. Frank Lloyd Wright Buildings (Unity Temple, Frederick C. Robie House, Hollyhock House, Taliesin, Fallingwater, Herbert and Katherine Jacobs First House, Taliesin West, Price Tower, Solomon R. Guggenheim Museum, Marin County Civic Center)
Definitely. I’m a huge fan of Wright, and have been to five of these sites. Other countries have similar UNESCO designations, such as the works of Anton Gaudi in Spain or the multi-country inscription for Le Corbusier designated in 2016.
4. Fort Ancient State Memorial (Hopewell Culture National Historic Park, Newark Earthworks State Memorial)
Meh. We have two major earthwork designations already – Cahokia in Illinois, designated in 1982 for the Mississippian culture, and Poverty Point in Louisiana, designated in 2014, for the Poverty Point culture. Now, the Hopewell are very important to North American culture, so if it becomes a site that’s cool. But that should be it for earthworks.
5. Mount Vernon
I guess? I mean, if we’re talking precedents of Presidential homes in Virginia, Monticello is a very unique space that Jefferson designed. What makes Mount Vernon special? It’s postcard perfect, but so far as I'm aware it's a pretty typical plantation house. Of course, it was George Washington’s home, and that’s swell, but does that make it globally important?
6. Serpent Mound
Nope. Potentially of the Hopewell types listed above. Either meld the two sites, both located in Ohio an hour away from each other, or just go with Fort Ancient. I’m for grouping them and calling them ‘Earthwork Legacy of the Ohio Valley’. That’s an inclusion I’d support.
As such I've expanded upon my original post, below, for eight sites we should definitely add of universal cultural importance. Not all are 20th century, one isn't even of the American era, but all I think are most deserving.
The Washington Monument in the largest stone obelisk in the world. The Lincoln Memorial and Thomas Jefferson Building represent the influence of the Beaux-Arts in America. The National Archives Building houses our Constitution, and Declaration of Independence. The Vietnam Memorial had global ramifications on memorial design, and the Smithsonian Institute was arguably the first of its kind. This grouping of buildings has become universally recognized as quintessentially American, grouped around the lawn and reflecting pool. All have been restored or preserved under the most rigorous standards.
San Francisco is an iconic, globally identifiable city. A city of incredible historic significance, it was originally settled by Native Americans five thousand years ago. Sighted by Sir Francis Drake in 1579, the Spanish arrived in the mid-1700s, and the city was the epicenter of the 1849 Gold Rush, bringing cultures from around the world to develop a unique, lively city. In the 20th century San Francisco was privy to many counter-cultures from the beats to the hippies to the gay culture the city is now famous for. Initially the seat of the United Nations, it is undoubtedly a global center.
This joint-party site is designed to venerate one of the few positive developments of the 20th century’s Cold War in the form of the Space Race. Gagarin’s Start, part of Baikonur Cosmodrome launched both the world’s first satellite, Sputnik, and first manned spacecraft, the Vostok 1. The Kennedy Space Center was the site of the first trip to another celestial body, as well as the space shuttle program that ushered in a new age of scientific discovery and international cooperation, in the form of the International Space Station serviced by both the United States and Russia.
The French Quarter (Vieux Carre) comprises a little less than a square mile of buildings dating from the Louisiana Purchase in 1803, with an array of cultural influences – Spanish, French, and Afro-Caribbean – that has played a critical role in America’s culture, and the world’s. Culturally the area may be most famous for Bourbon Street. Other historically significant districts besides the French Quarter include Treme, of tremendous global important as the site where jazz was invented as well as being a major influence in the blues, zydeco and Cajun culture generally. As the mouth of the Mississippi, it has played a very important role in the country’s military, economic, and cultural heritage including the development of steamboat technology in the 1800s by Robert Fulton.
New York is the largest city in America; located on the island of Manhattan it is considered one of the critical ports and cultural centers of the world. Initially settled by Native Americans the Dutch created permanent settlements in the early 1600s, with the English gaining control before the 18th century. It was part of the American Revolution, and was the city where Washington was inaugurated and Congress first convened. As the entrance for millions of immigrants in the 19th century the city grew significantly, and the city became an important focus of industrial developments. The growth of Wall Street in the 20th century lead to New York becoming a financial center, while movements such as the Harlem Renaissance, the East Village, and New York’s museums solidified a cultural influence. Perhaps most famously New York is home to Broadway – the leading light in global theater. Skyscrapers reached new heights in New York, as well as other engineering feats and designs into the mid-century. Finally the adoption of New York as the location of the United Nations cemented the importance of the city in world affairs.
6. Electric Pioneers, New Jersey and New York (Thomas Edison National Historic Park and Tesla Science Center at Wardenclyffe)
The workshops of Edison and Tesla capture the place where the modern world began to become electrified. Menlo Park saw the invention of the commercially viable light bulb, the phonograph, and significant improvement in the technology that allowed for moving pictures. Tesla's Wardenclyffe saw a giant batshit crazy "wireless energy" tower (since demolished) but the original building - designed by noted American architect Stanford white- still remains.
7. Ford Plants, Michigan (Piquette, River Rouge, Highland)
The assembly-line, the industrial, interchangeable parts revolution began with Ford's practices of industrial design. The three locations, from what was basically a small garage (where the first Model T was made) to the massive factory structures and plants that came to dominate the 20th century's reliance on mass market economies and production and mass consumption lifestyles.
8. Historic Jamestowne and Fort Raleigh, Virginia and North Carolina (Jamestown National Historic Site and Fort Raleigh National Historic Site)
Where it all began - the foothold that started the British on the path to becoming an empire. The origins of North America's colonization, and the origins of North America's slave trade. Unfortunately this designation would be rather difficult to get the U.N.'s 'okay' to preserve. We have located the two areas - but no real surviving structures from those colonies exist (at Jamestown there's part of a church tower from the 1600s - but that's it). Arguably it would not be a viable choice, therefore. If they're feeling generous, however, they totally should.