Lotr doesn't fit in mtg either. Mtg has its own universe with expensive lore. They've beaten the fans down so much that he new slop makes the old slop seem palatable. Also black Gandolf is a spit in the face of actual Tolkien fans.
People are trying to come up with all these psychological theories about why Marvel doesn't fit in MTG, when the answer is simple.
Superheroes are some cornball shit. Grown men in stupid costumes running around stopping bank robbers. Comics were invented as funny books for children, and no amount of Frank Miller gritty reimaginings changes the fact that Cap is a guy with a big A on his forehead throwing a frisbee.
Gandalf fits into MTG because LOTR's colorful goofiness isn't its primary driving trait. I can imagine myself summoning Gandalf to the battlefield because Gandalf feels like a real guy. Captain America does not.