The music and movie industries have tried their best to make it as hard as possible to consume legally bought media on anything other than what they want you to watch it on. Some of this is due to piracy concerns, some due to various contracts (you will only allow this movie on Starz for the first 6 months), but all of it is in their control.
Conversely, downloading a movie via bittorent will play on anything powerful enough to process an HD movie; downloading some MP3s will play on any device thinkable. The files require no complex codes to put into crazy websites, no additional software to install, no additional devices to buy. Being free is just icing on the cake.
Case in point: I bought Pacific Rim on blue ray when it was released a few months ago. Waited for it to arrive in the mail. Defeated its stupid plastic wrap. Popped some popcorn, turned the lights off, put the disc in my PS3 (the only blue ray player I own) and... nothing. It could not play the disc without connecting to the internet to download some new blue ray decryption keys or whatever. Never had this problem before, and sadly my PS3 is not connected to the internet. This disc was useless to me. Enraged, I downloaded an 18GB HD rip of it and had it playing on my $35 raspberry pi in an hour. Has this experience soured me on buying blue rays? Of course it has.