I think, and others have posted, that the reason is a prohibition in her Fox contract. Sounds right to me.
I dunno. If so, Fox is sure playing fast and loose by allowing other of their "talent" to ask, outright, who she is endorsing; or if she will endorse, etc. I think Palin is forbidden from having a regular paid gig at the same time she is a declared candidate.
I think she's signaling people to vote for Gingrich in the FL race, in as strong a terms as she thinks give her plausible deniability, and holding her fire for future races depending on what develops. If she comes out and endorses, she loses that flexibility; and also damages her value as a prognosticator. Why have her on any more? If she takes a position, she becomes predictable - probably hitting herself in the wallet, and also losing the venue.
There is no prohibition of Sarah endorsing anyone because she’s on Fox.....Hannity and Greta and many others like Pirro just now from Fox asks her all the Time for ‘when are you going to endorse someone?’ they would know if such things were prohibited and Huckabee has been asked about his endorsements—— no, there is something much bigger going on and I expect it to be made clearer in Feb. - her CPAC speech and heather things happening in that month. Hmmm