An old American tradition to not run a primary during a President's second term who is running for office. I guess it's supposed to help unify the party behind a proven winner or something. That's mostly it. Liberals love traditions, guidelines, and rules more than anyone.

There's some stupid adherence to precedent where we don't primary an encumbant because in the past it didn't work out well. So now we shut our eyes and pretend he isn't absolutely one of the worst candidates ever because we refuse to primary him.