There are all sorts of possible answers to that. The trick is figuring out which answer is right, or if there’s an entirely different explanation we’re all missing.
One theory, and I think the most likely one, is that Disney and Marvel think this will appease angry fans well enough in the short-term that they can get away with the forced retcon long-term. Disney and Marvel have been getting a LOT of hostility lately for treating the things fans and consumers care about poorly, and the backlash to what they did to Wanda and Pietro in the 616 adds more risk. As cool as female Thor, Kamala Khan, Captain Marvel and Falcon becoming Captain America are, it can only do so much to balance out the bad Marvel’s piling up.
Alternately, it could be people working at Marvel that don’t like the forced retcon either and really want the family restored. Secret Wars provides a good opportunity to use the family in its most iconic, cohesive incarnation without undoing the forced retcon. Perhaps those people are hoping that excitement and good will toward House of M will lead Marvel to undoing the forced retcon, or will at least keep the memory and ideal of the family alive long enough to be able to set things right at a later date.
Still another theory, perhaps the forced retcon was always meant to be temporary. We know it was done solely to avoid acknowledging the X-Men franchise and Wanda and Pietro’s ties to it during Age of Ultron’s release. This might be a way of saying “We know, we know, we pissed you off royally and you have good reason to be pissed, but we’ll fix it. Just give us a little more time, it’d look bad if we undid it right away.”
This last theory, I heard from someone else. Perhaps the forced retcon is such a radical change, it might violate some aspect of the film rights agreement with Fox. This would mean Disney and Marvel might get in hot water unless they have some version of them around that are still mutants and Magneto’s kids. I don’t know the specifics of the agreement, so I don’t know how possible or not that one is.
As always, theories are theories. They’re never certainties, they’re open to being disproven. This is just a handy list of ones I’ve thought of or have been suggested to me. 🙂