I think the difference is that you can have a very easy race on the road, which you can't really on an MTB (and expect a good result that is).
I always try and be pro-active and get into breaks and what not on the road, but usually get beaten by a sit in sprinter. Even then, I've never been anywhere near as wiped out as after an XC race.
You just can't do the sit in thing in an XC race - fastest rider wins.

