The big "money center" banks are at the pinnacle of the U.S. and global economies. Like sleeping next to an elephant, every single little move they make has an outsized impact on every dollar you own. No investor can afford to ignore what they're doing, lest they end up on the wrong side of a gigantic capital wave.
The regionals - banks with presence in five, 10, or more states - are different. They're important, don't get me wrong, but I watch them like a hawk for the same reason I listen to the results of my annual physicals: to make sure everything's working right, and nothing disastrous is looming out there.
That's because, as the history of the financial crisis clearly tells us, regional banks are where you find the very first signs of the kind of gross stupidity and borderline criminal behavior that can spread to the money centers.
It's like "monkey see, monkey do," except the sequence runs, "little monkey do, big monkey see, all the monkeys take complete leave of their senses, and the global economy crashes."