They are mutants, so changing them into ordinary humans means at least re-writing all the cells in the body, which is way more drastic a change than what we now know as gender transition.

Especially as for some of them, their Mutations are required for them to be compatible with life. You can't meaningfully change that without risking death or serious injury.

It's also rarely shown with much nuance. The cure more or less ends up being portrayed as a way to eradicate Mutants entirely, with the implication that it'll be mandated, rather than as a way to improve quality of life for those with mutations that could harm it, like the one kid who destroyed all organic matter in the radius of a few kilometres, rather than a weaker one to limit a Mutation so it won't cause issues, or remove it if they want.