A few factors at play:
1) The best NBA prospects aren't staying in college after 1 year. Blake Griffin in 2009 was the last "not a one and done player" (2 years at Oklahoma). A top 50-ish player can catch up and be on par with a One and Done future lotto pick, but that lotto pick is still earlier in their development. So they may just be a bit more raw.
2) There is only one ball. A lot of great college players thrive with the ball in their hands. However, they won't have the ball in their hands as much in the NBA because it's better to have it in a player that is a bit better than them (see number 1). If a player doesn't fit as well in an off-ball role, it's hard for them find their groove.
3) Type/Style - being crafty, being big and overpowering. This stuff works in college where players aren't as long and athletic, or don't have the same level of BBall IQ. Those types of ways of scoring just aren't going to be there as frequently or easily. Even just the timing and release of your shot, which explains why players that shoot lights out in college can't translate their shooting to the NBA (even though shooting is one of the more "transferrable" skills).
4) Defense - again, you can get by with effort and high IQ and knowing the system far easier in College than in the NBA. The Margins really tighten up, you can't run as much Zone because everyone on the opposing floor is a threat to shoot. If you aren't long and athletic, it's really hard, and once teams realize that they can start picking on you/aggressively hunting you on switches and their best ball handlers will have buckets for days, you're just done if you're not like Trae Young and averaging nearly 30 a game on the other end. I can't think of a quicker way to flame out of the League than being unplayable on the defensive end.
Most of the "not great" college players are ones that had intriguing attributes (length/athleticism) but just hadn't put it together up to that point before hitting the Draft. There really aren't that many that really fit that bill though, most NBA were really really good in college, if they weren't getting accolades it's often because they played on stacked teams ala Duke, Kentucky with other NBA prospects. There are 350-something college teams and a good number in the power six conferences. so there's a lot more room to have a bunch of really good college players at one time. Stuff like raw stats plays a big role in who is an All-American and not, but there may not be a ton (if any) of actual separation there between top players. It often just comes down to how they are utilized.