Well, the issue is that a lot of the trans research that had been conducted by then got burned by the Nazis. The modern idea of gender identity which is now widely recognized as the best possible explanation for the empirical outcomes we see was just being formed, and the dominant players in the field were still "sexologists" who decided whether someone was trans based on how attracted they personally were to their patient.

(John Money, who created the idea of gender identity in the 50s, was wildly off base about the specifics, which resulted in the tragic human rights violation of David Reimer, a cisgender man forced to live as a girl with crippling gender dysphoria. He also lied about the success of his forced gender reassignments, which resulted in routine intersex genital mutilation -- the one procedure that every single right-wing US bill banning gender-affirming healthcare for minors carefully excludes!)

Or at least that's my read of the situation! I could be wrong.