It is because of regulation that they are closing accounts without explanation. I used to run a debit card program with about 250K active accounts.

The account closures are driven by the BSA/AML process (and briefly by operation chokepoint). By law, we are not allowed to tell the account holder the reason for the account closure - this is to prevent scammers (and the overwhelming majority of closures were of shady accounts) from learning the detection pattern and changing their behavior.

So, don't blame the banks/card programs - blame the regulators.