Money in an FDIC-insured institution, whether it’s cash or not, is covered by the insurance. Money in a non-FDIC-insured institution is not. If an FDIC-insured institution started offering accounts denominated in bitcoins, they’d have to work out an agreement with the FDIC first.
This wasn’t a failure to be cash-like, it was a failure of Mt Gox to get FDIC insurance. And by the sounds of it, there is no way the FDIC would ever have agreed to work with an institution so poorly run as Mt Gox.
