I think it’s because if they change any of the Special Edition changes, then they’d have to change/rewrite parts of the current Star Wars canon too.
Back when his Twitter was still public, Pablo Hidalgo addressed the issue of the Special Editions as they relate to canon a few times. Basically, the way the Story Group sees it, all that matters for “canon” (i.e., what other storytellers are beholden to) is that Han and Greedo met in the cantina and Greedo died. The exact specifics of who shot first don’t really matter, at least from a canon perspective.
I also get the impression that a lot of folks at Lucasfilm would love to see the original versions released, but they don’t want to thumb their noses at Lucas’s clearly and repeatedly stated desire to have the revised versions be the only ones widely available.