I think it must be something about how high level mountain biking doesnt really look that different than mid-level mountain biking, as far as DH goes, and even a bit into freestyle, but when it comes to skiing, style is so important and there is more of a gap between pro and am, even if its just the size of jump and how smooth the tricks look, so while skiers will pay to watch the higher level skiing, bike films have to attract viewers in a different way, like telling a story or having incredible cinematography.
I'm waiting for a ski movie that even approaches anything that bike movies have been doing for the last few years. (see: Seasons, Follow Me, Life Cycles, and even the NWD series)
And I just need to mention the opening segment BLEW ME AWAY with all the bike factory shots.