Well the difference is..... 95% of the MCU movies are about the same TITLED characters in the form of mini trilogies.
Yes there maybe cameos/team ups but
Ironman is about
Ironman
Hulk was about
Hulk
Thor was about
Thor
Cap was about
Cap
Antman was about
Antman
And
Guardians did attempt to give each member a little shine and arc that you actually cared for so it was pretty much about
Guardians.
When the Avengers team up..... they all atleast essentially have a purpose and opportunity to shine.
Xmen you would think would be about Xmen, but sadly its just been initially a vehicle to propel Wolverine then later an Xavier and Magneto thing with everyone else far to the back
So..... to answer your question
I mean why is a tree and a talking rac00n more popular than Bishop, Colossus, and Kitty Pride
Martha