I can only guess he did not get a 1st or 2nd round draft grade. That's not necessarily due to his talent level; there are a TON of ultra talented receivers going pro this year.
I also think he has a lot of room for improvement as far as route running goes.
My general theory is that if you are 1st round you should go, if not you should stay. That said, I think that
@MrNubbz makes some good points. If it looks like the offense that you are a part of is going to regress, you might want to get out while the getting is good. I'm not sure that applies here, but it might.
My favorite example of this is Pepe Pearson of Ohio State:
In 1995 Eddie George rushed for 1,927 (5.9 per carry) yards and won the Heisman Trophy behind a stellar OL. The following year Pepe Pearson took over the starting job and rushed for 1,484 (5.0 per carry) yards. That SEEMED great, but in reality it wasn't. Any random college RB would have had about what Pearson had because nearly all of that OL that had blocked for Eddie George's Heisman returned. Any RB would have looked good behind that line.
Pearson decided to return for the 1997 season but the OL was gone. He only rushed for 869 (4.5 per carry) yards while splitting carries almost evenly with Michael Wiley who had 588 (5.6 per carry) yards.
Pearson definitely should have left after the 1996 season and staying around for the 1997 season cost him a TON of money because it exposed the fact that he really wasn't that great of a RB, he was just in a REALLY great position in 1996.