Partly because it's gotten way less unnecessarily difficult to access transition care

I like to point to another instance of something similar happening with left handedness. See, there was a huge stigma around being left handed that was very religiously ingrained (sinister being a term for someone being left handed), and children were mostly taught to be right handed because of this. Like, forcably switching hands for writing and eating and other activities if they used "the wrong hand". But, when this stopped being so poorly thought of and stopped being forced on children, left handedness SPIKED! You know, until it reached population levels, which is somewhere around 1 in 9. And then it stayed there.

Because, like, obviously left handed people didn't just appear out of nowhere. They were always there. They were just being forced to be right handed. And it was causing a lot of unnecessary trauma to force them to be right handed.

You got it pretty well, but I think it's also worth acknowledging just how difficult transition was back in the day.

Until the rise of informed consent in the mid 10s in the US, you needed a letter of recommendation from a therapist to start hormones. Depending on time period and therapist this could range from a sympathetic therapist double checking that you've thought everything through and it's not spur of the moment (still took 3-12 months and associated therapy fees) which is what I got, to a year or more of a therapist investigating to prove whether you're a "true transsexual" or not (best hope you're either transitioning to hetero or able to convincingly lie about it) and to ensure that there's no way for you to function without transitioning.

RLE (real life experience) was the default until falling out of fashion in the 10s in the US. It was basically you have to live full time as your preferred gender (as the doctor sees it) often requiring passing for cis or meeting some attractiveness standard, for anywhere from a few months to 2 years, after which point you could be prescribed hormones. Hell, in the mid 10s a friend got denied a letter of recommendation because she didn't wear a skirt and makeup to the therapist. RLE was sometimes compared to hazing.

For a long time therapists also had weird expectations on trans people like that we should go "stealth" ie start a new life where nobody knows we're trans. I think it was the 90s when a controversial study was done where the researcher followed trans people long enough to become trusted only to find that once we trusted them we stopped being overly performative of our genders in front of them.

Oh and this isn't even touching on the difficulty of finding a doctor to prescribe hormones, which still isn't easy.

And that's not touching on the social side. You're looking at violence, especially of the intimate partner variety, and disowning. You're looking at your career prospects likely being torpedoed and possibly being pushed into sex work (or at the very least for it to be a real concern). You're looking at a really difficult life, and this is previous generations so by the time someone figures this out about themselves they may be married with kids.

When I started transitioning in 2015 there were still trans people giving the advice to wait to start until you were deciding between transition and suicide. Transitioning was the best decision I ever made, and yeah I don't blame them for that advice. When it's that hard, you need to know there's no going back before you start.