Because some things are a choice, and some things are not. Some things can be turned off and some things cannot.
Gender (and gender identity), along with race, are things that cannot be unshed.
The language you choose to speak, what music you listen to, and whether you choose to share your sexuality, while they are differences, they cannot be instantly detected.
The natural heuristic we use drawing upon our experiences in an American society have a unique relationship with gender and race. It is especially because those things are non-choices and instantaneously recognizable, that they are the most important to address first.