To begin with, you can find a lot of unreliable information on the Internet.
It seems to me that the reason is rather banal, because our society as a whole has a negative attitude towards LGBT people. And the older generation (and I think there will be a lot of doctors among them) grew up in a more conservative time, when it was not customary to talk about transgender.
Which doctors should be aware of this? As for me, there are a lot of psychiatrists - a schizophrenic has a place in a straitjacket, and this is clear to everyone. Don't be crazy - a minority.
Doctors can say, for example, that transgender people do not live long. (this is not true) That you must be a stereotypical, hypertrophied person of a certain gender, although this is not even applied to cisgender people. Or that you have to live for a certain time in the right field, while not giving you help and access to hormones and how expensive doctors can socialize, while looking the wrong way? Or they believe that this is a perversion and perversion, or even derailment. Do not hesitate to say how they do not like such people right in the face and make a face at the same time. Or not to understand hormone therapy, where does transgender come from. To say that gender identity and sexual orientation are the same thing. Yes, a lot of things they say and don't know.
Why are so few LGBT activists aware of transgenderism? It's just a quiet horror when you read their comments even on this site. Is it hard to read basic things on the internet?