Populist Reform of the Democratic Party

The election of 2008 was unusual in this regard. From the beginning of the twentieth century forward, only three Senators have been elected President: Harding, Kennedy, and Obama. All the other winning candidates had significant executive experience. Many had, like O'Malley, been governors, including four of the last six winners (Carter, Reagan, Clinton, and Bush43). The others had served as President or Vice President (which also covers Gore if you count him as the winner in 2000), plus the two unusual cases of Hoover (head of World War I relief efforts and then running a Cabinet department) and Eisenhower (Supreme Commander of the Allied Forces in Europe).

So, although it's not a requirement, it does seem to help. (Also note that, when Obama won as one of the three exceptions, both major-party candidates were Senators without such executive experience.)

It helps get votes and it also probably helps the winner do a good job. A governor has experience in making appointments and in dealing with a legislature to get things done. (In O'Malley's case, however, he was dealing with a friendly legislature, so his experience won't carry over perfectly to what a Democratic President is likely to face in 2017.)