Hanskey asks “How come governors get presidential nominations more often than people in other offices?”
That's a good question, and political scientists have been talking about it for decades. Barack Obama notwithstanding, the last time a sitting senator was elected was 1960, with John F. Kennedy, and Warren G. Harding in 1920 before him. Bush, Clinton, Carter, and Reagan were all governors, and Bush Sr. and Nixon were Vice President, though Ford was the House minority leader and Johnson and Kennedy were both in the Senate.
Now, you have to keep in mind that the current system has only existed for about sixty years; it used to be that the party leadership decided the nominee, and the few primaries that states held decided a very small portion of the delegates. Heck, until the turn of the century or so it was considered gauche for presidential candidates to campaign themselves, whether for the nomination or in the general election.
To be fair, it is a little unseemly to talk yourself up as the best person for your party to put forward, but whatever.