I am going on the assumption that MTB racing is newer than road racing. So, there is one thing. There hasn't been enough time for pros to get conceited, distant, and rich.
I have also found this MTB phenomenom with Tri pros, too. Triathlete pros are so approachable. But, the sport is very new, too. The tri races I have been too have an area for the winners circle blocked off (i.e. where the pros sit). But, it is usually a small tent with a few very uncomfortable chairs. They soon pop out and mix in with the crowd so much that it is hard to find them. I was sitting at the finish line watching a tri race end, once. I saw all the pros gathering for their winning interveiws and such. Then, they walked out right passed me and joined their friends and families. I sat down on a curb and began talking to my friend. Next thing I knew, I heard this foreign accent over my shoulder. I turned around, and there..sitting right next to me was the 6 year veteran winner of the race, pro triathlete Alec Rukosuev. I didn't talk to him. I guess I was too nervous. I looked around and saw one of the age groupers talking to Karen Holloway. Everyone was so laid back. It was really cool.
I have noticed a certain snobbery with road biking in general, though. Maybe it is just where I live. But, usually when there is a large ride, there are several groups of rich snobs all decked out who snort at you and look down on you...until you pass them. 🙂