Black Panther 2 has a specific mission that might have impacted Marvel’s original plans for the story. Marvel has to introduce a new Black Panther after Chadwick Boseman’s untimely death a few years ago.
Marvel already confirmed it would not recast the role. Therefore, the Wakanda Forever plot had to be modified to account for the real-life events. Marvel will need time to explain the death of King T’Challa, Boseman’s popular MCU character, and explore the fallout of that event. Moreover, the film will have to show how the next Black Panther becomes Wakanda’s Black Panther.
That’s on top of advancing the story of Wakanda and providing the audience with additional pieces of the greater MCU Phase 4 puzzle.