It was Spider Man and the Hulk at first..then 1975 with Claremont changed everything. The X Men became a force of nature..sales..eventually Video games...the shows...the movies. Left the Avengers, FF in the dust..like the Infinity Gauntlet story was great but the Age of Apocalypse was absolutely legendary...Fatal Attractions when Magneto ripped Wolverine and Cable apart was huge....I mean that X-men were more popular than the avengers. From 80’s since the Chris Claremont run. Some of the individual avengers were popular due to tv and movies but as a whole they weren’t fukking with X-men from like 81 until the early 00’s when Civil War took place in the comics.
The x-men are still more popular in comics, they just don’t have big movie tie ins right now