Because we need a fast receiver at the #3 (slot) and Ginn (even Hixon) fill that roll great. We don't need Nicks if we get a receiver round 1.
With the type of production a rookie WR typically has we would actually be the same if not worse off than just keeping LaFell. And that's assuming we hit on a WR that will do well. A good vet, whether or not it's Nicks, is needed