With Fast Travel, what's the point of a Mount, really? If Oblivion's the meterstick we're going by, then a Mount wouldn't be able to go all the places you could go (A Steep Incline! The Horse's one weakness!), you'd be vulnerable to attacks (And in a game that features Rocket Launchers, that's never a good thing), and by the end of the game, you'd end up being faster then the mount anyway.
Mounted Travel really would add nothing to the game beyond less reason to explore between destinations. And what do you do in situations like the D.C. Ruins where a mount/vehicle literally cannot go?