One of the main reasons seems to be because various research on trans issues has been slow or completely stopped in previous years due to pressure from individuals both outside and inside the community. As a result, many people, even professionals, still don't know a lot about all of the nuances involved. An endocrinologist may determine that someone is "female" based on hormone distribution but disregard a complete physiological view, which would include something like differences in skeletal frame and mass or differences in grey and white matter between sexes.
Also, a lot of the research on trans was presented from the school of Psychology and unfortunately, most people in the field are aware of the long history that Psych has had with subjectiveness and forced, hurried, or missed diagnosis. All of this has led to a lack of knowledge which has led to a mess of assumptions and misinformation, it's why you see the general population doing everything from conflating and confusing intersex with trans to professionals mixing up biological and psychological specificities (even some of the ppl on OH who treat science like a dogma one minute turn around and propagate pseudoscience the next).
This is the same kind of conversations that we've had concerning age and sports, disability and sports, prosthetics and sports,ethnicity and sports, diet and sports, stature and sports, and so on but for some reasons, trans is off-limits? People would rather someone get physically harmed than have to participate in an actual discussion and revisit certain decisions that have been made; the conversation often dies before any information is shared.