I find this rather strange given that the English-speaking population of the rest of the world combined is much greater than that of the US (even though the US is the largest single market for books in English) and the fact that there are lots of extremely talented writers in other countries. I would have expected that at least a good number of Amazon bestseller indie authors I see would be people based in other countries, but it hasn’t been the case. I suspect it might be because books self-published by authors only tend to be visible within their own Amazon geographical sphere and are typically not displayed to US customers unless they are residing in the US.
While a great many people speak English as a second language, and can thus enjoy reading a novel in English, the majority of people for whom English is the native language live in the US and the UK. (There's also Australia, Canada, South Africa and several others. However, Canada's population, for example, is half the UK's, and 1/10th of the US's. Australia's is even less.)
Now, a person for whom English is a second language might or might not be proficient enough in English to write in it, as opposed to just reading. Even if they are proficient enough, they would have to choose to write in English rather than their native language. Some writers might indeed choose that - Lavie Tidhar is one example. Others are writing in their native language, and the reason you don't see them is that (I assume) you're only looking for literature in English.
Now, I write in English, even though my mother tongue is Hebrew. One of the reason I made this choice is that the whole of the Hebrew-speaking population is about the size of the population of London. Percentage of fantasy/sci-fi (my genre) readers among them - about what you'd expect. So the readership I can expect if I write in Hebrew is miniscule. Now, a person writing in Chinese, or Spanish, or Russian, wouldn't face such a problem, right? So there's less cause for them to pick English over their native language. But that very reason is also the reason you don't see many people from my country as successful indie writers - there's less of us overall.
America has all the e-readers
It's about money and culture. The USA is one of the few nations where people can afford to have things like Nooks and Kindles and devices for doing nothing but reading books, which makes eBooks a lot more accessible. In most countries, the market penetration for e readers is not as high. According to Amazon, North America was the largest consumer market for e-readers in the last few years, with a market share of 72.42% in 2011 and 68.48% in 2015. Europe and China ranked the second and third markets with the market share of 13.86% and 7.82% in 2015.
Why is that so important? Because while smart phones have eaten into the e-reader market, they don't make optimal devices for reading an electronic book. Most people just can't justify owning that many different electronic devices though. The e-reader market predicts who are going to be the most successful Indy authors because while publishing has changed dramatically in recent years, the part that has really been decentralized by the ease of self-publishing has been the e-book market, not nearly as much the print book market. While Indy authors can and do get physical printed books onto the shelves of brick and mortar stores, it is significantly more complicated and difficult to do than the easy distribution offered by eBooks. Most large bookstores want a book to be offered by a distributor which can guarantee money back on unsold copies, for example. That is a huge hurdle for an Indy author.
That means that the e-book market, which is the epicenter of all activity for indie authors, is absolutely dominated by American consumers. Both the affluence of the USA as well as copyright laws that facilitate electronic book distribution make America the center for e-books. It is common sense that if you really want to appeal to (say) Germans, it is very helpful to be German yourself. You can make your readers relate to your characters through shared cultural connections, commonly understood cultural shorthand, etc. So American writers have an advantage in reaching American readers just because of shared culture.
Perhaps we will see the kind of technological revolution that will fully decentralize publishing (physical in addition to electronic publishing) in coming years, and many aspects of this paradigm might change, but for now, successful Indy authors are a pretty American phenomenon.