If I were a complete cynic, my thought would be that there's some really nasty stuff in his past that he's afraid would come out in a Presidential campaign, so he's not going to take the chance.
He's still relatively young, isn't he? Because my next thought would be that someone (Hillary) has told him it's in his best interest to sit this one out and wait a bit.
Warner's pretty young. And yeah, it's plausible that Hillary told him to wait his turn. And in reality, his daughthers will be adults if he waits for the next cycle.