You don't want to rush your guys into an environment they aren't ready for. Every year in the NHL draft, about only a couple players are ready to play in the NHL. The only two ready this year are Hedman and Tavares, and rightfully so.
Some go to College for plenty of reasons. One, if you're a late round draft pick, say 5th-6th... you can't be feeling too high on yourself to make a living in the NHL at that moment. So people want to go get an education, where they can improve on their hockey play, at the same time.
A lot of guys need to continue to be developed and improve on their game before you try putting them on an AHL/NHL roster. Either that be skating, shooting, defensive game, etc.
There are reasons why people become late 1sts/2nd rounders... is that it's going to take time to develop them into an NHL hockey player. There aren't even a lot of players that play in the AHL the following year they get drafted. You take baby steps, can't rush these guys. Be patient with them, and they can turn out to be great.
Example : Scuderi...5th round draft pick for the Penguins. Went to Boston College for 4 years, then into the AHL, then eventually in the NHL with the Pens. Do I need to say how well that turned out for us?