(1) They got lucky. Netflix made the good deals early on. After a few of these, the deals simply snow ball.
(2) Netflix has a captive audience created through their DVD rental service. Netflix gives us DRM'ed content we can view online and we applaud it because we already love getting those red envelopes in our mailbox.
(3) Their distribution network is superior and is the best to deliver all this content.
To me (2) seems the most likely. Perhaps is a combination of all 3.