Los Angeles specifically has Thai Town in North Hollywood, which is a large area of concentrated Thai immigrants (although it was intentionally named that way pretty late, in 1999, as a sort of tourist attraction) as a way to attract development and interest to the region. LA was the first focus of Thai cuisine and immigration in the U.S. For this part I’m going almost entirely off the book “Flavors of Empire”, so rather than paraphrase I’ll just quote the author in an interview directly:

“One is that it starts not when Thai immigrants come to L.A. but when the United States goes to Thailand during the Cold War in the 1950s and 1960s. And by the United States, I mean the U.S. military, Peace Corps volunteers, Fulbright fellows. Those U.S. citizens come into contact with Thai food in the 1950s and 60s, write Thai cookbooks, introduce it to suburban housewives back in West Los Angeles, and that gets the ball rolling.

And then within a decade or two you have a huge migration of Thai students coming to study in the Los Angeles area and around the country. They are the first ones to open Thai restaurants to feed themselves and to feed other Thai students. Then they decide to try to sell some of this food to Angelenos.

The other big turning point is the opening of Bangkok Market in 1971. Before the opening of the market Thai people are cooking quote, unquote Thai food, but they're really using Chinese ingredients as substitutes.

So, they have soy sauce but they don't have fish sauce. They don't have any kaffir lime leaves. The Bangkok Market was the first to import Thai canned goods, Thai produce, and Southeast Asian produce. They tried to grow this produce in California. Ultimately, they end up in Mexico growing 90 percent of the Southeast Asian produce that comes into the United States. Those three moments gave rise to the Thai food scene in L.A. in the late 1960s.”

To the point of the other southeast Asian countries you listed: Thailand and Vietnam are simply much larger than Cambodia or Laos, and culturally much stronger. Much of Cambodian history from the decline of the Khmer empire is the story of looking to the large, powerful Thai kingdoms to throw off the yoke of the Vietnamese (as embodied by the famous master’s soup fable).

Adding to this history is that Thai food has specifically benefitted from a policy from the Thai government that intentionally funds these restaurants around the world, called Gastrodiplomacy. In 2001, the government created the “Global Thai Restaurant Company” with a goal of creating more than 3000 Thai restaurants worldwide. Banks were established to provide loans to start Thai restaurants in other countries. The public health ministry published a supportive manual. There was even a push to train chefs within Thailand to send abroad. Overall this strategy worked; and we saw a huge proliferation of Thai cuisine (and subsequently it’s culture) across the world. But you, living in LA, probably notice it more than anyone, as due to a combination of geographic and cultural factors it was the first foreign epicenter of the Thai culinary movement.

Sources-

Padoongpatt, Mark. Flavors of Empire: Food and the Making of Thai America. 1st ed., University of California Press, 2017. JSTOR, http://www.jstor.org/stable/10.1525/j.ctt1tqx733. Accessed 7 Jun. 2022.

Zhang, Juyan. “The Food of the Worlds: Mapping and Comparing Contemporary Gastrodiplomacy Campaigns.” International Journal of Communication, https://ijoc.org/index.php/ijoc/article/view/2847/1316.

The interview: https://www.splendidtable.org/story/2019/01/10/how-thai-food-took-over-america