Elsewhere in Kevin Feige’s interview with Variety, he explained that the first half of the MCU’s decade-long existence was spent building towards The Avengers.
So why did it take so long after the establishment of the MCU?
Once those relationships were established, and the MCU could move towards the events of Captain America: Civil War, it became increasingly manageable for them to introduce an outsider to represent the rift between Tony Stark and Steve Rogers (Chris Evans).
Of course, many members of the audience have been clamoring for a Black Panther movie since the 1990s, if not earlier, and Hollywood icons like John Singleton even tried to pitch (decidedly different) versions of the T’Challa story long before the genesis of the Marvel Cinematic Universe.