There are all kinds of reasons for it. Most every player has to go through a period of adjustment that comes with being new and having to learn how things are done at a completely different level. You have to put in your time and your hard work, and earn your spurs, so to speak. Very few come into a college program and become immediate star performers.
When players ahead of you either depart the program or go down with an injury, an opportunity is created. But you have to be ready when the moment arrives and not squander the opportunity.