I haven't made a study of the differences in fees across credit unions and banks. There may be people in the room who will have done so.
I would say two things. First of all, every financial institution has a different business model. Some of them emphasize different things—customer service, convenience, a range of products, etc.
The other thing I would point out is that in the federal sphere for banks, there is an obligation or an agreement, depending on how you look at it, to provide low-fee accounts. Each bank has an offering for a bank account that has low fees, that is quite low-cost. It's part of the federal regime to ensure that this option is available for consumers. By no means do all of them choose the low-cost option, but it's always available.