You need to be a Prime member to buy many Marvel and Star Wars discs now.
According to a report by The Verge, Amazon is now selling several Blu-rays, 4K Ultra HD Blu-rays, and DVDs for Disney's Star Wars and Marvel franchises as Prime Exclusives. Though the discs are still readily available from other retailers, if you want to buy them from Amazon you will need to sign up for a Prime subscription.
It's not yet clear how long the Prime exclusivity will last, but it seems to be covering all of the major releases for both series, including The Force Awakens, The Last Jedi, Rogue One, Ant-Man, The Avengers, Avengers: Age of Ultron, Captain America: First Avenger, Captain America: Winter Soldier, Captain America: Civil War, Guardians of the Galaxy Vol. 1, Guardians of the Galaxy Vol. 2, Iron Man, Iron Man 2, Iron Man 3, Doctor Strange, Spiderman: Homecoming, Thor, Thor: The Dark World, and Thor: Ragnarok.
As of press time, the only main exceptions appear to be the original and prequel trilogy Star Wars titles, and The Incredible Hulk. Likewise, some DVD and 4K versions of certain titles, like the 4K Ultra HD Blu-ray for Spiderman: Homecoming, don't appear to be included yet. Amazon has sold other discs as Prime Exclusives before, but never on this scale. The move to push buyers toward Prime memberships with these films is likely being made to capitalize on the upcoming theatrical release of Avengers: Infinity War and Solo: A Star Wars Story.
With that said, it looks like third party sellers are not impacted by the decision, so some discs and used copies for certain titles can still be ordered from affiliated retailers through Amazon. A current Amazon Prime subscription costs $99 per year or $11 per month.
Source: The Verge