Maybe because they said they weren't sure they could do them well enough that they would be in the final game :whistling:
True, at E3 they was confirmed but no mounted combat like in Oblivion, this makes horses outright dangerous as in Oblivion, only worse as Skyrims enemies are not matched to your level so you can get tails more dangerous than mountain lions, you might also ride into dragons who will be faster than you, not that fighting a dragon from horseback is a good idea but a chance of getting attacked by dragons while you are trying to outrun a bunch of bandits.