Initially, it was Apple's goal to make a large amount of movies available through Apple TV. 1,000, in fact, by the end of February. Saying that they fell a bit short of that goal would be a severe understatement, as they have managed to scape up only 351 titles for rent. Apple says they are pleased with growth as it stands, but obviously such a limited selection can only hurt them in the long run.
Why the limited selection? Many reasons, for sure, and it is open to speculation as Apple is making little comment. One reason might be technical complications, such as we saw with a recent software update being delayed. They might also be facing difficulties with movie studios and getting permission to rent out their films. Early adopters of the service might be dismayed at this slow speed, but Apple is not commenting on exactly what the holdup is.