The one I hear about the most is service. Large, do-it-all banks don't have a lot of motivation to care about you and your $100,000 bank account & half-a-million mortgage. They've got corporate accounts to service against which yours is a mere rounding error. A smaller local bank will find you to be big, valuable customer to keep around.
This is a general economic principle. To put it into an HN context, this is why Google, Amazon, Facebook, Microsoft, and such aren't the only tech companies. When they sit down to spend a dollar, they do an analysis and put it into the place that will make $1.60 (tech companies still have absurd return ratios compared to the rest of the world even after the last year). It doesn't make any sense for them to put it where they will make $1.30. It doesn't matter how big they get or how much money they have, that analysis always holds true.
This is why Google shuts down so many things. Even being profitable isn't enough for them, it has to be wildly profitable and at scale to compete with "making ads better". It is also why it is perfectly rational to be distrustful of anything Google puts out and fear it being shut down. It is not a transient corporate culture thing, it is a systemic issue with where their profit comes from. And yet it is rational for them to float out various things as probes to see if there's some huge profit for them to tap into, even as total long shots.
Meanwhile, other smaller companies can happily survive and thrive on that $1 -> $1.30 return, and then themselves be too big to worry about a $1->$1.13 that a startup may thrive on.
There are some other reasons why One Big Company isn't actually a practical outcome, but this is one of them, and the one relevant to this discussion. It applies to tech companies. It applies to the retail industry; this is why Wal-Mart and the fancy boutique downtown import shop can still coexist, even today. It applies to the auto industry, which is why your local auto dealer may service a local business' 10-car fleet but there will be another company servicing huge fleets. And it applies to banks.
This does not mean a large bank is obligated to be negligent of you, it just mean that there's a pretty strong pressure that is hard for them to resist. Strong internal leadership may push the consumer branch to be friendly as a sort of advertising mechanism for the rest of their bank, because you never know what individual will be in charge of directing a large corporate account in the future. But they'll be vulnerable to the next MBA to come along and cut that cost and boost short term profits, and who will have moved up before the long term costs come in.