No idea, they don't seem to have made that public. But seeing as more and more outside US processors stop catering to US businesses and customers it's not hard to see why.
Edit: Funny thing is that they didn't even bother sending an email, found about it from one of the forums i follow...
The way they put it on their weblog, their partners in the US stopped. It does seem to have anything to do with "[...] more outside US processors stop catering to US businesses and customers [...]".
Never heard of Payza until now. That much will affect me.
2. It's practically impossible to run a payment processor in the U.S. without getting funds frozen. PayPal is as successful as it is in part because they built a giant machine-learning monstrosity to automatically detect and deal with fraud (and it's full of false positives, but they consider that just a cost of doing business on the U.S. Internet).
Edit: Funny thing is that they didn't even bother sending an email, found about it from one of the forums i follow...