Almost all credit unions require you to put up your other accounts as collateral when applying for one of their credit cards. Thus, if you default on the card, they can take your savings and apply it toward what you owe them. In contrast, very few banks require this sort of collateral when opening a credit card account.
Why do all banks not require this? It would seem to be just good business practice.
I guess this is one reason we haven't heard of any credit unions failing.