I would be very worried about banking security if there was only one bank and it was run by the government. Obviously, that's not the case, banks are private companies and there are thousands of them constantly competing for each other. They have a strong interest in tight security to remain trustworthy. They're also heavily regulated, probably even more than the voting system, and they're subject to financial auditing. I'm not an expert but would estimate that for these reasons banking is overall more secure than electronic voting.

I could be wrong, though. As far as I know, hardware companies nowadays cannot even be reasonably sure that the chips they use don't contain backdoors.