You’re assuming that they’re honest about their reasons, which is not the case.
The main message of the film (besides being a 2h ad for Mattel) is gender equality and female empowerment.
This threatens these people more than homosexuality, but I imagine even for them saying “we don’t want our women to even imagine a world where they’re not subservient to men” would be a tough sell.
Far fewer people are willing to stand up to defend the LGBT community so they’re a convenient scapegoat (‘conservatives’ will always find a group small enough to target in order to push their unpopular policies, it just works).