I heard a good conversation on this with Dan Patrick and Joe Theisman yesterday. Theisman noted how difficult it is to get a player to come on to a team in the middle of the year. They don't know the schemes, they don't have the play calling memorized (it's like learning a different language), and they don't have the timing/continuity with the other players. Teams usually want players in their camps early. And, with the Salary Cap, trades often result in one team taking a big hit.
For all those reasons, it's much different than Baseball where you can plug a guy into a pitching spot or another position and there really isn't much to learn in the way of timing, etc. He's going to be the same guy. The same is true in basketball - timing with teammates helps, but the guy can still get his shots off, play defense, etc., even if he may take some time getting to know his teammates and getting into the offense or defense. It's still an easier adjustment.