Many in Washington and on Wall Street favored a more centralized central bank, either dominated by bankers or politicians, but Congress passed the law requiring a decentralized organization with eight to 12 reserve banks to disperse authority. That tension between centralization and decentralization has waxed and waned over the past 100 years and continues even today.
In the end, Congress and the Reserve Bank Organizing Committee opted for 12 Districts. Their size was based on the number of banks in their Districts and their relative size in the economy.