It wasn't that the banks didn't have enough money for it, it's just they literally didn't have the money. Look at
poker stars, they were able to pay out all of their US players with no issues. That's because they didn't spend player funds. And I don't believe it was a Ponzi scheme, even if they were accused of it.
What they basically did was take the money that was supposed to be reserved for player withdrawals and paid themselves with it. I think they just got in over their heads since it was so successful, they basically cash advanced to themselves with the expectation that future business would cover for it. Honestly, I'm sure the longer it lasted, the worse it would have been. Greed would have gotten the best of them and they would have continued to do it.