I think it's true that a lot of crypo investors come from the US, at least in terms of wealth. But I don't think it's "most". In the past at least, I think more has come from Asia (especially China and Korea). And regulations in the US make it difficult to safely operate a cryptoexchange without risking difficult-to-predict legal troubles.

For example, it's hard to know if the coins that you are listing might be considered securities by the SEC (because the rules are very vague and the SEC doesn't just say which coins they consider securities and which ones they don't). And if the SEC decides that it's a security, then you're supposed to have a special license to allow users to trade that coin, and if you don't have that license, then you could be fined for allowing trading on your exchange. Even if you have that license, there still can be issues in which of your users are allowed to exchange that coin, depending on if they have enough wealth to be considered an "accredited investor" (just means someone rich enough that they are allowed to speculate on risky investments).

A similar problem can arise with respect to money transmission laws in the US. In this case, if you accept US customers, then you have to make special reports to the US financial crimes enforcement (FINCEN) organization if transactions are "suspicious" and exceed certain amounts of money. It also requires you to keep all sorts of additional records about your customers.

These rules, plus the concern that the rules and/or interpretation of these rules can change at any time, make it more difficult and scary to operate a cryptocurrency exchange that accepts US customers. This is also why Bitfinex and Binance have stopped accepting US customers.