Re: What I don't understand is
Perhaps because the US does not have a parliamentary system of government?
Governors have had a lot of success running: Carter, Reagan, Clinton, G.W. Bush. Senators elected directly from the Senate (i.e. without a VP stint in between) have had a good deal less. Most recently there was Kennedy, and before that Harding. Off the top of my head, I can't think of anyone who was elected directly from the House of Representatives.
If you haven't been elected before it usually works out best to have been a general, it is true.