Of course IQ is involved in any activity but I don't think there is anything so unique to mountain biking that it would self select for a specific type of intelligence, beyond the basic traits inherrant in most outdoor adrenaline sports. I imagine it appeals to thrill seeking and a certain immortality mindset that the young generally possess and then winnows that down to those who have it as a more base mental outlook as age and injuries eliminate many who would otherwise grow out of that phase.
beyond that, those with quicker reflexes, a higher pain threshold, success economically or socially, and access to trails probably dictates who stays and who falls off.