Mostly because it was the 2nd pick in the draft. You need a playmaker in the top 5 or some "Sure" thing if you are going off a playmaker position, somebody who touches the ball or rushes the passer.
But to boot, this guy was a development project and odds were the Rams would be developing him for his 2nd contract with a different team. It was pretty clear he wasn't going to be much of a player for a few years.
NFL - Not for Long. You can't take an asset like the 2nd pick in the draft that typically can get you an entire draft or a pair of 1's and 2's for a development project to hold your team hostage for a few years and pay him as an intern to develop for some other team on his 2nd contract.
The change in decision making these past few years on valuing assets has been so refreshing. Maybe Snead learned or maybe he was a puppet for Fisher and is now finally getting to make his own decisions.