Given the relatively small number of people who suffer from gender dysphoria, it doesn’t make much sense to have a category devoted to people who do not suffer from it. “Cisgender” people constitute the vast majority of the population. In the absence of evidence to the contrary, shouldn’t we just assume that people don’t have gender dysphoria, much as we might assume that people don’t have other ailments, like tuberculosis or AIDS? We don’t have a word that specifically describes the condition of not having each particular disease or disorder, so why do we have one for people who do not suffer from gender dysphoria?
To “normalize” the transgender experience, of course. To use the word “cisgender” is to tacitly affirm that being transgender is just as normal as not being transgender, even though this just isn’t the case.