Because
troon surgery ban vs no ban is not an "all else being equal" thing.
Currently, troon surgeries nominally exist to treat "gender dysphoria", which means society defers to troon ideology. The worst and dumbest are disproportionately (but not reliably -- look how many troons troon after childbirth, and consider that Uncle Ted, a genius mathematician, nearly got caught) Darwin'd, but the normies who successfully pass this extremely low hurdle live in a worse world as a result: they're poorer, more demoralized, produce less culture, and may be having fewer children.
Were I a dicktator, step 1 would be to ban troonism; this would classify troon surgeries with other cosmetic mutilations (udders, buttocks, bottocks, nose reassignment, etc). Step 2 is to have those banned, too, for general moral reasons, and
because they compromise the medical profession (which should be about making the
patient better, not giving the
customer what (s)he wants).
But you (and unfortunately I) live in democratic societies. In yours, troon ideology is hard to ban or restrict because of free speech. (In mine, women'd be up in arms if their right to die from botulism was denied.) You have to chip away at troon privileges. Banning surgeries "because they're ineffective at treating dysphoria" would
- save public money
- inject some evidence into the medical profession
- remove the main financial incentive for troon propaganda and pseudoscience
- give (dumb) young people more time to grow a brain