Originally Posted by
Hybrid
In fairness, Marvel did kneecap the X-Men (and the Fantastic Four) by not allowing them in Marvel media such as animated series, merchandise, video games and what-not, and I know the X-Men haven't played a pivotal role in a Marvel wide event in a long time. It's telling that as soon as the buyout became a thing, this ended, with Marvel Ultimate Alliance 3 featuring them. Likewise, the Fantastic Four were cancelled and remained so for three years, and didn't return until the buyout was imminent.
So while the X-Men were a thing in the comics, they were just getting by. Sure, we had X-Men stories, both in their titles and mutants did appear in the General MU, but to deny that NOTHING was happening over this is absurd. I've seen people who don't even know the X-Men share the same universe with the Avengers in the comics, and that's criminal. As is acting like the Avengers are the core of Marvel and the X-Men exist outside of it. When the X-Men and the Avengers crossed over, it was the X-Men who were treated as the stars and the Avengers as second fiddle. Again, see Onslaught.
How much this applies to the comics landscape is debatable, but Marvel definitely had an interest in promoting the characters they had the film rights to at the time. Same with the Inhumans getting promoted suspiciously similar to the X-Men, because Perlmutter wanted an equivalent for the MCU, not anticipating the buyout. When the X-Men film rights returned, notice how the Inhumans basically vanished.
It was not just "Marvel was developing properties besides X-Men and Spider-Man", it was "Marvel was promoting their biggest properties for the MCU, at the expense of what wasn't available for movies (X-Men and Fantastic Four)". Spider-Man will always remain an exception due to being Marvel's mascot, and a huge cash cow even without the film rights. I'm not saying they totally built this up in the comics, but I can see how it would happen. I personally think it was a process of multiple factors. The Avengers getting pushed to the heart of the universe may have been to give Marvel their own Justice League, but when the MCU rolled around they went in with it at full force, while downplaying the X-Men in the process.
My two cents.