I don't think anyone who has a somwhat decent shot at being president just arbitraily gives it up.
I don't think there's a politician alive that doesn't dream of being president. That's why I think there's more to this.
Could be affairs (male or female), could be shady money dealings, could be drugs. Who knows? I don't think M.Warner is telling the truth, the whole truth and nothing but the truth.
Or as has been speculated, maybe Her Thighness promised him something to get him to withdraw from the race. That wouldn't surprise me either.