I think this has more to do with the economics of pirating movies versus music.

When Netflix started, people were not used to torrenting (or Napster-ing, or Kazaa-ing, etc) movies. They were already at the mercy of Blockbuster, so being at the mercy of Netflix instead wasn't that big a deal. Pirating movies is more popular today, but people are still generally OK with having a limited movie selection.

Whereas by the time iTunes Music rolled around, people were already comfortable with having the whole world of music at their fingertips. You can't undo something like that easily. So the only way forward has been to try to have everything.

Perhaps the most interesting part about all this is that there was never (to my knowledge) such a thing as a store where you could rent records, tapes, or CDs -- Blockbuster for music.