They have slow update paths because they've spent a lot of money on in-house middleware that they don't feel a need to update, and sometimes those systems don't work so hot on the new browser. Faced with a choice between updating old middleware and not updating the browser, they choose to wait on updating the browser. The longer they do that, the more work the middleware needs to be "up to date", and the worse the problem becomes. Definitely not isolated to banks.