How did this happen

A couple things:

• As gays started gaining mainstream acceptance, and particularly in the last few years when they've gained widespread support and legal protections, the focus of lobbying efforts started to shift to the T, because these established orgs needed to be "doing activism" to keep bringing in the donations and keep their jobs.
• A lot of people realized that in hindsight gay people should have had these rights all along, so people didn't speak up earlier because they were afraid of being seen as hateful in the way we see homophobes now.

So the result was you had this modern trans activism pushing for self-id as a legal standard, and "affirmation only" approaches to adolescent gender dysphoria cases. Also they started telling kids that GNC behavior meant you were trans, and again you didn't have people shutting that down because they were afraid of being called bigots.

You can see it now even in 2020. Rowling has done nothing but explain her reservations about men who haven't really transitioned, and GNC and autistic girls being pushed into transitioning despite basically every study that exists showing the vast majority will eventually grow out of it, and she is still getting called a bigot that wants trans people to die because of it. She's too rich and established to be cancelled, but other more vulnerable people are probably afraid to speak out still for this reason.