I think this is very dubious. Barry Goldwater, George McGovern, Bob Dole, and John Kerry were all sitting senators nominated for president during those forty-eight years. Yes, they all lost, but this has very little to do with their being senators. There is no reason to think that if they were governors they would have done any better. (Kerry after all came a lot closer in 2004 than Dukakis had in 1988...) Besides, saying that 2008 was a fluke because the top three candidates were all senators ignores that nobody knew in advance that McCain would be the GOP candidate--he had to win the GOP nomination over such non-senators as Giuliani, Romney, and Huckabee.
Also, there was a forty-four year gap between governors getting elected president (1932 to 1976) and a sixty-year gap between *sitting* governors getting elected president (1932 to 1992).
The truth is it's hard to discern an actual pattern because there aren't that many presidential elections and political patterns change. Insofar as you can discern one, it seems to me that big-state governors may have a leg up in the nomination system because of tighter control of their own state parties and greater on-paper appeal, both because their job is superficially more like the presidency and because their absence from the national scene makes them a "new face."
Many senators, especially those who have been in office for a long time, may not have run a competitive race for years and are therefore rustier and have weaker connections with major campaign consultants. They also have a weaker in-state network of supporters. If they can overcome that - which they generally can, provided they are prominent, have run some tough races, are relatively appealing, etc. - and if they can get the nomination, there doesn't seem to be any actual hit in the general election.
I don't buy alternative explanations, like the argument that their voting record can be used against them. That's true of plenty of governors (who often served in state legislatures or Congress beforehand), and in any event, governors have to sign every bill that comes before them, many of which can prove controversial, as Michael Dukakis found out during his campaign.