It's nothing more than keeping up with the evolution of the cycling sport. It's been in the "modern" Olympics since the start. Road racing was added as it became more popular, same as mountain biking, and finally BMX was added. BMX has become a rather large sport around the world so it's only natural that it should be added to the games. Also pure BMX racing I wouldn't consider an extreme sport. Half-pipe, vert, big air stuff, yeah that's extreme and that's what the X games are for but pure racing, not extreme at all.
The other thing to consider is the money generated from the games. If they ONLY stuck to the classics as you call them, the games would take 5 days to run and be over, and who wants that? At least with BMX it IS a physicaly demanding sport that showcases an athletes abilities. It should have been added long before ping pong, or badmitten were even considered a sport which I still don't think those should be in there.
