He quit for a VERY GOOD REASON- his daughter was dying and he needed to be with family. Get your facts straight.
For whatever reason, I had forgotten that and I suppose it makes me seem like an insensitive guy, for which I apologise.
That being said, he was frustrated by life in the Senate. See:
http://www.opinionjournal.com/editorial/feature.html?id=110009798
Alas, my central point that he seems to have little executive experience still stands.
D