Why? Perry and Huntsman are non-factors. Gingrich is an actual threat (for now at least).
An endorsement that brings them loads of positive press and momentum in an early primary state would be a big deal for either Perry or Huntsman, both of whom are polling only somewhat lower than Gingrich in New Hampshire. (It would probably be an even bigger deal for the Huntsman or Perry campaigns since both of these candidates are currently receiving little attention.) Both may be non-factors in New Hampshire at the moment, but they're still potential threats to an easy Romney victory.
Gingrich's momentary strength shouldn't be overrated as an advantage before it becomes clear that the field has become less volatile. In fact, the best part about this endorsement coming now for Gingrich is that it disrupts a news cycle that otherwise would be dominated by stories about him taking millions in consulting fees from a public company.
More importantly, no matter who the UL had picked, this endorsement plays into the dominant Anyone But Romney narrative.
