We're not. For one thing, Fox had partial control over their Spider-Man release slate during that initial time, it was not entirely a Marvel choice. And Spider-Man is an important character, but Marvel has always been measured and careful in their rollout of new properties, not overloading them. They did not sideline Captain Marvel or BP for Spidey, they simply took their time and are still doing so. Spider-Man is a proven seller and they wanted to get him set in the universe without having to go through the arduous process of reestablishing a third Spidey in the public consciousness in recent years - they did that. And as for Black Widow, the only reason there wasn't a film sooner is because old right wing Ike Perlmutter vetoed it for years, until Feige got him thrown out of meddling in the MCU completely.
I'm not seeing any evidence that they are "forgetting" the characters that got them here. CM2 and BP2 are both actively going forward.
Both have a place on the slate. The X-Men don't, yet. I know these properties at least as well as you do and I'm simply not seeing what you are seeing. YMMV.