I'm no political philosopher, but I'd say, for example, the Preamble of the U.S. Constitution says the government's purpose is to “promote the general welfare." Phenomena like bank runs harm the general welfare, so the government has an interest in preventing or mitigating them.
I agree that bank runs harm the general welfare, and the reason why we can make that claim is because of the material relationship between the banks and the general population, as individuals participating in the economic system in order to ensure their own welfare.
But if we look at other sorts of more complex financial instruments, the sort of thing that's inaccessible to an average citizen, then why are they protected by the state as well? The obvious answer would be "well, banks invest in complex financial instruments, and the state wants banks to be protected". But regulating financial instruments isn't necessarily the only way of ensuring that the banks' service to their customers remains stable, it's just one possible path.
