Because your objection should not be based on how much you agree with a competition. That is not the argument. The argument is that men cannot become women. They cannot ' woman' better than women can.
But if a competition is inherently sexist and you believe it's demeaning, and you'd like to see it die a death, wouldn't you care less than if it was something which lifted WAGs up?
I mean over the years things like wet t-shirt competitions, Butlins used to have nice leg contests, children's pagents, things like that. They shouldn't die because fugly men enter and win. But I can see women thinking that another nail in their coffin isn't a bad thing.