While it may seem obvious that Scarlet Witch would never join the X-Men due to the unforgivable crime of mutant genocide she committed in House of M, she actually revealed why she didn’t join the team of mutant heroes long before that traumatic event.
House of M was a Marvel Comics event that shone a villainous light on Scarlet Witch after years of the character acting as a hero following her initially-evil debut. In House of M, Scarlet Witch rewrote reality to make mutants the superior species over humans, with Magneto acting as the unchallenged ruler of the world. However, at the end of the series when she felt overwhelmingly unappreciated by Magneto, she uttered the now infamous phrase, “No more mutants” and in the blink of an eye, eliminated most of the mutant population. This event made Scarlet Witch a pariah in the mutant community until very recently in Marvel Comics continuity, though it wasn’t the first time Wanda deliberately separated herself from mutantkind.
In Avengers #16 by Stan Lee, Jack Kirby, and Dick Ayers, the founding members of the Avengers are getting worn down by the job and wish to take an indefinite leave of absence. However, they couldn’t just shut down the Avengers Initiative as that would leave the world vulnerable to any number of attacks that are too great for any one hero to handle. So, the Avengers decide to look for replacement heroes to take their place while they take some time off. In that effort, the Avengers place an ad in the paper, calling for any and all superpowered or highly-skilled individuals to try out. Scarlet Witch and her brother, Quicksilver, answer the call.
Before thinking about joining the Avengers, Scarlet Witch and Quicksilver were members of the villainous team known as the Brotherhood of Evil Mutants led by the infamous Magneto. However, Scarlet Witch and Quicksilver were never evil themselves, and only helped Magneto because he saved their lives years earlier. While initially tethered to a life of villainy by a life-debt, the Maximoff siblings decide to break free of Magneto’s control as they could no longer abide by their villainous lifestyles. While it seems like the two should have joined the X-Men if they wanted to be heroes as atonement for their crimes against the heroic mutant team, they explain why that wasn’t an option for them. In this issue, Quicksilver speaks for both he and his sister when he explains their reasoning for opting to join the Avengers over the X-Men, saying, “I wish to forget that we are Homo Superior–born with powers denied to ordinary human beings! But, the Avengers might accept us without caring that we are different–without always reminding us–we’re mutants!”
Both Quicksilver and Scarlet Witch wanted to use their powers for good, and they wanted to feel accepted by society while they did so. The X-Men, while a prominent team of heroes, have always been outcasts because of the fact that they are mutants, and that is a stigma Quicksilver and Scarlet Witch wanted to separate themselves from. Even though Scarlet Witch’s initial disdain for mutants may have led to the events of House of M, that event wasn’t the driving factor behind her inability to join the X-Men as Wanda revealed the real reason she never wanted to be a part of the team many years earlier.