@drgnslayr
It's the AAU ball, oddly enough.
Long time ago, most players would only get 25-30 games of action in, basically their HS season, plus a few summer camps here and there. In the late 70's and early 80's, AAU teams started traveling, but again, the tournament schedule wasn't that heavy (maybe one tournament a month in the summer) and two or three camps for the elite players. By the late 90's AAU teams played all summer, with summer camps sprinkled around for not only the elite players, but also for solid players.
That means that an elite HS player in the 1950's or 60's would have played, by the time he got to college, maybe 125 games max, with maybe 15 of those being against top notch talent. By the 70's and 80's, that was maybe up to 175-200 games, with probably 50 games against top talent. Today, the best HS players play as many as 100 games each summer against top talent from around the country. They don't have to adjust to the jump in talent level as much because they have seen the top talent on the AAU circuit for the last four years traveling around.
Think about a player like Perry Ellis, who followed something of an old school model in his development. He played AAU, obviously, but didn't travel as heavily as some and certainly didn't spend as much time at the elite camps as many players do these days. He was a historically great Kansas HS player. But the first year of his college career he had to figure out the speed and athleticism of the college game. At Wichita Heights, Perry played maybe 5 games out of the 100 in his HS career against a D1 caliber player that was taller than 6-6. That won't prepare you to jump straight into D1 ball.
Add to the sheer amount of games that many players begin lifting weights now in the 7th or 8th grade, and that many high schools have a full fledged weight training program. This means that in addition to getting more games in, these players are more physically prepared for college than players from even 15 or 20 years ago.
Go back and look at the highlights from the McD AA games from the 80's and early 90's, then watch today. The players just look more physically developed. It's a startling difference.
You also have to remember that the vast majority of the juniors and seniors playing college basketball are non-elite players. As a result, the truly elite freshmen and sophomores can dominate because they are just better.
For example, If Anthony Davis were in college right now, he would be a senior and would probably be unleashing an absurdly dominant season on college basketball right now, given that he is averaging 25 points, 11 rebounds and 3 blocks a game in the NBA right now. He averaged 15/10/4 at Kentucky as a freshman. Let's say his numbers improved by 10% each year. He'd probably be throwing up a 20/13/5.5 right now. You think UK's current bigs are good? How would Davis look in college right now? It would be flat out unfair.
The college talent level is trimmed from the top every year. The best freshmen are likely better than the best sophomores, who are likely better than the best juniors, who likely are better than the best seniors, because for the most part, their best peers are already NBA players. There aren't very many truly elite seniors in college basketball. There are some elite juniors, but only a handful or so. Most of the truly elite players are freshmen and sophomores, and those are the guys that dominate.