Again, i've gotta agree it probably has to do with the bigger paydays and risks the further you go on in your career because if you look at prospects, they fight a lot more often than the established stars. Two reasons I guess. 1) because they don't need as long to train for the usually tame competition they're put in with and 2) The more established you are, you're normally making more money and the more dangerous your opponent usually becomes, thus perhaps longer time is required for training to be at your absolute best.