The real reason the Bank of America CEO suddenly quit.

People around Bank of America should have known something was up when its beleaguered CEO, Kenneth Lewis, returned from his August vacation with a beard. For a buttoned-down Southern banker like Lewis, growing a beard in Aspen is the equivalent of a full-on existential crisis. And so the announcement of his retirement on Wednesday wasn't such a big surprise. (Here's the farewell memo.)

Analysts may cite several reasons for the seemingly abrupt end to the career of a self-made, homespun banker: the midcrisis acquisition of Countrywide Financial, big credit losses at the bank's core operations, and the hornet's nest of troubles the bank acquired when it purchased Merrill Lynch last fall. To these, I'd add another reason for Lewis' downfall. He wanted—no, needed—to make it big in New York.

In America, there's banking—taking deposits; making loans to homeowners, small businesses, and large regional companies; and serving on local boards. And then there's Banking—lending to hedge funds, investment banking, structured finance, and serving on the boards of famous cultural institutions. You can be a very successful and large banker in Charlotte, N. C., which is where Lewis' Bank of America prospered. But you can't be a Banker unless you're in New York. It's one thing to be the chef at the best French restaurant in Chicago. But the rewards—psychic, cultural, social, emotional—are much greater if you run the top French restaurants in Paris.

For Lewis, getting into investment banking was the equivalent of seeking to open up a bistro in Paris. But unlike some of his CEO colleagues, such as JPMorgan Chase CEO Jamie Dimon, he wasn't born into the brokerage business. Born in Meridian, Miss., and a graduate of Georgia State University, Lewis lacked the social and cultural connections that frequently smooth the way for Wall Street careers. Lewis didn't summer in the Hamptons or stroll over to Temple Emanu-el for Kol Nidre or pop up to Allston for Harvard Business School reunions.

And so Lewis and Bank of America had to buy their way into Wall Street. Under Lewis' leadership, Bank of America spent a lot of time and money trying to build up a presence in New York, hiring bankers and erecting a tower near Bryant Park. But Bank of America couldn't push its way into the top tier. And a year before the world fell apart, Lewis began to think better of his attempt to make it here. "I never say never, but I've had all the fun I can stand in investment banking at the moment," he said in the fall of 2007.

A year later, he was back. As Merrill Lynch choked on losses tied to mortgages, Lewis saw a chance to buy the nation's largest brigade of stockbrokers and gain control of an iconic investment banking franchise. At the press conference announcing the deal, I asked Lewis what made him think this foray into investment banking would be more fun than the last one. Lewis crowed about Merrill's quality and headed off quickly to be interviewed by Maria Bartiromo of CNBC.