On one hand there were internal factors: Marvel spent a long time building up a brand, plus Disney figuring out a good formula that blended good acting (Robert Downey Jr. in Iron Man) with special effects and gimmicks that straddle the line between stupid and awesome (the S.H.I.E.L.D. heli-carrier)

One big external factor has been China as a market. That one is definitely on the outs

https://movieweb.com/chinese-censorship-in-the-mcu/