The main issue is that the MCU only really had half of Marvel. The Avengers, Captain America, and half of Spider-Man. The X-Men and Fantastic Four were both owned, until recently, by Fox. They have now regained the X-men and FF, but too late to do anything with them before the Avengers arc finished. They also suffered from the fact that actors nowadays have far more freedom of action. Under the old system, actors played the parts they were told to play and when the studio decided, the role was given to another actor. Nowadays, Evans and Downey can pocket their cheques and sail off into the sunset without the risk of being told "You'll never work in this town again!", and the studio is nervous about recasting their iconic roles. Since the Avengers without Iron Man and Captain America is about as much use as a Sherlock Holmes story without Holmes and Watson, they were sort of stuck. So they produced some fillers with less-known characters while they started work on the other two major franchises. A shot in the foot while waiting for a shot in the arm, so to speak.

As for Disney, they've been on the back foot since the late '90s and their failure to acquire the Harry Potter franchise. They already knew they'd never get LOTR, old man Tolkien had made it clear to his heirs and assigns that this must never happen. But I would guess that they'd counted on getting Potter, as the acknowledged master-adaptors of chldren's literature, and thus have a secure decade or so of guaranteed blockbusters. But JKR was having none of it. Disneys' attempt to rival HP with Narnia crashed and burned. Relying increasingly on other production companies for success (Pixar, Walden Media and Jerry Bruckheimer), Disney themsleves have done nothing of worth between The Emperors' New Groove and Frozen, and nothing since the latter They needed those Marvel films! So they milked them mercilessly and ended up saturatingthe market.