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Why are there many small banks and more banks in the U.S.?
As an addendum to PeterK's answer, once upon a time, there were many Savings and Loan Associations (S&Ls) that acted as small banks, accepting savings deposits from people and lending money for home mortgages to local residents. Some of these S&Ls were chartered Federally with deposits insured by the FSLIC (similar to the FDIC which still insures deposits in banks) while others had State charters and used the State equivalent of FSLIC as the insurer. To induce people to save with S&Ls instead of banks, S&Ls paid higher rates of interest on their savings accounts than banks were permitted to do on bank savings accounts. Until 1980, S&Ls were not permitted to make consumer or commercial loans, have checking accounts, issue credit cards, etc., but once the US Congress in its wisdom permitted this practice, this part of the business boomed. (Note for @RonJohn: Prior to 1980, S&Ls offered NOW accounts on which "checks" (technically, Negotiated Orders of Withdrawal) could be written but they were not checks in the legal sense, and many S&Ls did not return these paid "checks" with the monthly statement as all banks did; writing a "check" while pressing hard created a carbon copy that could be used as proof of payment). In just a few years' time, many S&Ls crashed because they were not geared to handle the complexities of the new things that they were permitted to do, and so ran into trouble with bad loans as well as outright fraud by S&L management and boards of directors etc. After the disappearance of most S&Ls, many small banks (often with State charters only) sprang up, and that's why there are so many banks in the US. Mortgage lending is a lucrative business (if done right), and everyone wants to get into the business. Note that 4 branches of Bank of America in a Florida town is not a sign of many banks; the many different banks that the OP noticed in Maine is.