While Marvel Studios are certainly thinking about how to bring the X-Men into the MCU, the WandaVision series can justify their absence with a plausible explanation.
Phase 4 of the MCU is meant to introduce the multiverse and alternate realities. As we know, the series WandaVision will be the first to inaugurate this new plot, but it could also have the mission of introducing the X-Men within the universe. The characters were originally owned by Fox, but now that Disney has bought her out, they can finally land in the Marvel movies. As far as we know from the synopsis of WandaVision , Scarlet Witch will live a happy life with Vision and their twins , which is simply impossible since the android is dead. The MCU's Most Powerful Witch has therefore created an alternate reality in which she lives a utopian life. Is this also how the series will justify the absence of the X-Men from the universe during all this time?
If we trust the comics of House of M , Scarlet Witch suffered so much from Vision Loss and her twins that her grief distorted reality. His father, the mutant Magneto, and Professor X therefore did their best to help him overcome his grief, to no avail. Wanda's brother Pietro also advised her to create an alternate reality in order to survive, a parallel universe in which all of her loved ones would be alive. In this biased world, also lived a horde of mutants led by Magneto. But once he learned that all of this wasn't real and that the idea for alternate reality came from Quicksilver, he killed Pietro.
Very angry, Wanda then decided to destroy this reality by blowing only three little words: “No more mutants”. In short, the original timeline then resumed its course, crushing the majority of existing X-Men in its path. If some elements of the comics have been changed for the Marvel series, WandaVision can take inspiration from this plot to justify the absence of the X- Men. We could be explained that Wanda accidentally wiped out mutants in this world, but no one in the MCU remembered that reality . Remembering this, the mutant abilities of the beings involved could manifest again, bringing the X-Men back to the original timeline. If at present this theory is not confirmed, we just have to wait 4296559 the release of WandaVision on Disney to be sure!