Another answer: because regular banks are awful to deal with. They don't have nice APIs to just move money around. They apply velocity limits at surprising times. They can terminate your account suddenly for opaque reasons. They charge high regular fees and even higher "gotcha" fees if you make a mistake. They can take back money sent to you for up to 90 days, but often can't get back money you sent to crooks even if…
> They can terminate your account suddenly for opaque reasons. Isn't one of the requirements for being a "bank" that you can't do stuff like that?
US law requires banks to file a suspicious activity report if they see unusual activity in your account that may be associated with money laundering or criminal activity. What qualifies as suspicious activity is not well-defined, but can be as simple as large unexpected transfers into or out of the account.
The law also requires banks not to tell you about it. They don't necessarily have to close your account, but most will, especially after several reports, because of the risk of failing to comply in the future. And since they're not allowed to tell you why they closed the account, you end up with opaque reasons.