Because there's about 1% of people who are trans in the US [1] and most other populations, on top of that come a further about 1.7% people who are somewhere on the intersex spectrum [2].
Democrats aren't willing to throw literally millions of people under the bus. Republicans aren't just willing to cast them aside - they are knowingly driving trans people to extinction.
[1] https://williamsinstitute.law.ucla.edu/publications/trans-ad...
[2] https://www.ohchr.org/en/sexual-orientation-and-gender-ident...