I think there a many factors which may or may not be contributing to a decline in snowbirding.
Just the cost alone of new TV's and newer unites, many of us are on fixed budgets and things are just not getting any cheaper. Not to mention the many problems some have experienced and reported on, here on this forum.
Then there is the fact that we are not getting any younger and medical problems are starting to catch up with us.
As we get older traveling is not as enjoyable as it once might have been, many have nightmares about getting past Atlanta ;).
Then again many of us, "have been there done that". The novelty of the adventure or experience may have lost some of it's original appeal.
For some, extended family issues keep us close to home and for some its the loss of traveling companions which distract from the pleasure of travel.