It has nothing to do with the US and regulations.
One must think before blaming them of a simple question, how would it be possible for every exchange that opens up to keep going ?
It's not, so just as other businesses exchanges lose volume, lose clients an go bankrupt.
The US based company behind Bittrex US filled for bankruptcy half a year ago, this is Bittrex global which should not give a damn about those since they target international clients, but, they have no more clients at all, so the global branch goes after the the US one. We have 200 spot exchanges, some, like 90% of them need to go!