The real reason is that he wanted the challenge of running and winning and that's all. He will do nothing, repeat nothing, while in office except to prop up the reputation of his previous stint as governor. It's the same reason that Jimmy Carter will go on any talk show and talk to any media person about his failed presidency. He has become pathetically distanced from reality.
The same thing will happen to Jerry Brown. His run for office was just an ego test which he passed. He will soon ask the same question that Robert Redford asked in "The Candidate" – "Now what do I do?"
What he does won't be pretty.