The passion, plain and simple. No matter what team (or sport), the SEC fanbases take it seriously and root their teams on, no matter what. You just don't find it collectively anywhere else. I know we are all rivals and hate each other, yadda yadda yadda, but no matter what anyone else tries to convince me of, when it's bowl season I'm pulling for the other SEC teams to pound their opponents.
It's a southern thing, no matter what we have between us, we will all stick together against the world. People from all other regions of the U.S. make fun of it and put it down, but deep down they all wish they had it. It's why when they come to the south, they never leave.