If you listen to Chris Matthews on MSNBC, you'd think the answer was that Romney and his pac outspent Gingrich by a large margin, with a torrent of negative ads against Romney. This was the same reason Matthews (and other commentators) gave for Romney's evisceration of Gingrich in Iowa.
Except ... Rick Santorum beat Romney in Iowa (or came in just a few votes behind Romney, when Matthews offered his assessment), and Santorum spent next to nothing on ads in Iowa compared to Romney. And Romney outspent Gingrich not only in Florida but in South Carolina.
Which means the ad expenditure theory in primary wins and losses just doesn't add up.
What does add up is this: Gingrich pummeled Romney in the two debates prior to the South Carolina primary, and seemed weak and even befuddled in his two debates with Romney prior to Florida. In terms of a simple experiment, you can get much better evidence than that.
What this means for the future is ... well, it depends upon which Gingrich we see in upcoming debates. The pre-South Carolina Gingrich in debates could and may well get the nomination, whatever Romney spends. The pre-Florida Gingrich in TV debates doesn't stand a chance.