With the exception of some shows doing occasional Thanksgiving episodes (EVERYBODY LOVES RAYMOND seemed to do one every season), there's really only two relatively obscure Thanksgiving specials that actually come to mind when it comes to thinking of holiday specials in general: A CHARLIE BROWN THANKSGIVING, and that Hanna-Barbera special from the 70s with the talking squirrel who was at the first Thanksgiving (I keep forgetting what it's called).
I've said it before, but I think one of the problems too is that many holidays are celebrated almost purely for the secular aspect: Christmas has Santa Claus who delivers toys to good little boys and girls on Christmas Eve, not to mention you've got the lights and the trees and the decorations; Halloween has kids dressing up in costumes and going out trick-or-treating for candy, plus again you've got lights and decor and such; even Easter has the Easter Bunny who hops all over the world delivering baskets of goodies to kids, and there's the tradition of coloring eggs and having easter egg hunts and such... but, is there really any kind of a secular aspect to Thanksgiving? Like heralde just pointed out, Marcie does say that Thanksgiving is more than just eating, but that's just about the only thing people think of when it comes to Thanksgiving, and that's kind of hard to secularize. Even Santa Claus actually has a historical background: he's based on a real-life Saint named Nicholas who would bring a sack of toys to poor children on Christmas Eve. Valentine's Day is the result of a Saint named Valentine who was executed on February 14 because the king discovered he was secretly marrying people, as the king wanted all the able-bodied men he could get to fight in war, and they couldn't do that if they were married. Not really sure what the story is with the Easter Bunny, but whatever.
But yeah, I guess it's hard to turn the idea of European settlers being thankful for a new land and a feast that the Natives shared with them into a big secular commercial racket, which is probably the main reason why Thanksgiving always gets the short end of the stick as far as holidays go.