The factor that I think makes the big difference is the rise of the NAN. Thanksgiving is often associated with images of stereotypical native Americans, which would probably be heavily frowned upon by the NAN. Additionally (this isn't fluff) but it seems reasonable to me that a group of NAN terrorists might try to ruin the Thanksgiving celebration at some point.
... except for all the "wanna-be tribals", which I would think the NAN would dislike even more. There ought to be a "Tribal Poser" NQ.
Here's my theory: Thanksgiving probably evolved over the decades to be less about the pilgrims and their first harvest feast, to be more about family time. We sort of see that happening in a lot of parts of the US anyway. With the other two major changes to the country, it seems reasonable to me that the American states would have adopted a tradition more akin to Canadian Thanksgiving.
I suspect it's more related to the dwindling portion of the population that has anything at all to do with any kind of "harvest" in the first place. It's all owned, run, and controlled by Ag mega-corps in the 2070s, and their profit margins probably don't allow for much of a bounty.
The wage-slaves should be focusing on a more marketable holiday, anyway... like Xmas, or Chanukah, or (even better) the Winter Solstice. Your employer knows what you buy and how much you spend, and if you don't shop for gifts for the people they feel should be important to you, you'll be spending an afternoon in HR, explaining why (and probably being sent for counseling).