Because you want to play right away and/or have a chance to be drafted. At son's P5, I would say there will only be about 2-4 true incoming freshmen that will get any significant playing time this year. That is the case on most teams in the SEC and almost every competitive 4 year school on every level. So many players will go to a juco and get bigger, faster, stronger and get to play right away. For many it is also about getting locked in academically at a smaller school before making the jump. I don't think the average player is ready for the academic rigors of 4 year college sports activities, social activiities/distractions, and the academic stuff.