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.
The biggest single reason that governors tend to do better than members of the House and Senate in presidential primaries is likely a legislative record: senators have to be ready to defend every vote they made, whether the bill passed or not. For a governor, it really only matters if a bill reaches his or her desk, and it's even more rare for a governor to comment on a bill being considered than it is for the president to do so.
Now, you might think that the lack of damning votes would also help a mayor or a member of the federal cabinet, or even a state cabinet. This is more or less true, but except for New York City mayors tend to have very low name recognition, as do cabinet members, and anyone who isn't an elected official isn't likely to have an organization to work for reelection or nomination. The last person to be nominated without history as an elected official was Wendell Willkie in 1940, who lost overwhelmingly to Franklin Roosevelt, and the last person to be elected was Herbert Hoover, who was Commerce Secretary in the 20s.
Another subsidiary reason is that governors don't have to split their time between DC and their home state, and while everybody needs to fly around fundraising for the presidential nomination, that does make a difference for building ties in your state, and having as much time as possible not be spent traveling. It may also be a factor that needing to be elected every four years rather than six means the state electorate is more familiar with you, although most states also have term limits on the governorship (most often 2 4-year terms), so that leaves a rather narrow window for running.
Ultimately, of course, all this is speculation, and a lot of it is likely due more to the individual candidates than the specific office they currently hold or most recently held. The nominations of Barry Goldwater and Ronald Reagan were about the ascendancy of the conservative wing of the Republican Party, of which they were leaders; Reagan spoke of the need for smaller government and lower taxes, not his record in California.
Frankly, most people who run for President never even make it to the Iowa caucus, and those who get nominated have reached beyond their office to build alliances with folks across the country—nobody does it alone. George Bush and Bill Clinton didn't get nominated or elected because they were governors, they used the organizations they had built over the years to win, and they did it better than their rivals.