Mounted Combat consists of ALOT of new animations...
Each weapon type has it's own animation now so this would mean for each weapon that already has a new animation attack they would need to or probably need to give each mounted combat weapon a unique animation as well.
It's hard to add mounted horses combat in unless it was something they started working on at day 1.
I don't think mounted combat will be in, unless they decide to halfass it which todd basically said he doesn't want to do.
Personally I'd love to see mounted combat, even if it wasn't as detailed as unmounted combat.... Simple swings, and jabs, and Proctile animations would make me happy, but when you have a game that is going for A+ in every field it leaves little room for things that will alter the rating by a game reviewer. (not saying thats the only reason that they wont do it, but I beleive Todd is a perfectionist.)