Trading volumes do not allow them.
The United States has a lot of bitcoin enthusiasts, and if they want to provide services to these Americans, they must comply with the United States regulations, which impose restrictions on dealing with many countries.
Bitpay service was fined for supporting purchases of people from Iran and North Korea.
I guess I can see that if they are based in the USA or any nation friendly to the USA. But what would prevent Iran or North Korea from opening a crypto exchange and allowing anyone to trade? The US can't really fine them or do much about it right?