There are things that big banks do for customers that small banks can’t, because of economy of scale. Compare the web site and online services of Bank of America to that of your local credit union. I love credit unions (I’m a member of two), but they just don’t have the economies of scale to justify spending nearly as much on technological (and other) infrastructure costs like a big bank does. In the wholesale banking space, there are literally hundreds of bank services that big banks offer, but small banks can’t, mostly for the same reason. Wholesale customers want a bank that they can do all their financial business with.