It has nothing to do with the United States alone. The biggest and most important reason .com is king is because of it's position in history and money spent on promoting this one TLD.
When the first 3 gTLD's were introduced .com was the only domain to be used for profit making companies. All over the world, but mainly in the US, companies spent billions of dollars adverting the .com brand as if .com it is the internet, as if it to say there is no other website but a .com website. At this point in history, we, the consumers, have been trained to use the .com. Ebay, Amazon, Monster, Google, even DNF, is .com. While you and I in here might realize the potential for other TLD's the consumer has to be trained to use other TLD's they're not just going to type in another TLD, the majority are not computer savy and they will think it will not work any other way. So, until other TLD's have billions spent on advertising they will never overcome the .com as the internet, as a brand.
Think about it, really, how can any other tld compete? Ask a non-computer relative about a website. It will be a .com.
Again, this isn't just my opinion but the current state of affairs. I would like to see other TLD's succeed and broaden the base but getting my mom to type something other .com fries her circuit, she doesn't get it. LOL
Cheers,
Paul