The movie studios really don’t want you to buy their movies on discs (DVD or Blu-Ray). They would much rather you buy (or rent) a digital file via iTunes or Amazon or the like. Once you buy a disc it’s yours forever, but a file is ephemeral, and could be encumbered with “digital rights management” (which the studios love almost as much as money).
Take Mockingjay: Part 2 as an example. Fan sites have resorted to writing lengthy “buying guides” to help people figure out when and where to buy the version that they want to buy. If you want all of the deleted/extended scenes from all four movies, you have to buy the 8-disc box set. But if you do that you don’t get the extra bonus features for Mockingjay: Part 2 that are available only at Target! If you want all of those bonus features you have to buy the disc from Target. Why? Who knows… Lionsgate and Target are friends? Of course the disc doesn’t come out until Tuesday, but the digital online version of the movie? It’s out already.
If you think all the options for buying Mockingjay: Part 2 are confusing wait until next month when Star Wars: The Force Awakens comes to disc (a few days after it’s available as a digital online file, of course).