Why it's not on Disney Plus: Universal Pictures owns the distribution rights to "The Incredible Hulk." The studio coproduced the movie with Marvel Studios. Unless Disney strikes a deal with Universal, the movie may not appear on Disney Plus. For diehard MCU fans, this might be disappointing. But for casual viewers, the movie isn't an essential entry in the franchise (actor Edward Norton was recast with Mark Ruffalo in "Avengers").
"Spider-Man: Homecoming" (2017)
Why it's not on Disney Plus: Sony owns the film rights to "Spider-Man" movies and can keep them as long as it releases a movie every five years. The studio struck a deal with Marvel Studios in 2015 to allow the character to appear in the MCU. Sony would retain distribution rights to the character's solo movies while Disney would earn a percentage of box-office gross and all merchandising revenue. Sony and Disney struck a new deal this year for actor Tom Holland's Spider-Man to star in one more solo movie and appear in one other MCU movie. If Disney wants "Homecoming" on Disney Plus, it will have to strike another deal with Sony.
"Thor: Ragnarok" (2017)
Why it's not on Disney Plus: "Ragnarok" is currently available to stream on Netflix. It's expected to leave Netflix next month, so it shouldn't be long before it's streaming on Disney Plus.
"Black Panther" (2018)
Why it's not on Disney Plus: "Black Panther" is currently on Netflix, but is expected to leave the service in March.