I'm pretty sure that before they made it a central topic, most Americans didn't care about transgenderism, which after all affects a very tiny population. Especially compared to other issues, such as paid maternal leave, for instance.
I assume you are aware that "they" are the Republicans? The Harris campaign avoided talking about it whenever possible, while Republican groups spent $200 million on anti-trans ads.
Why restrict it to the Harris' campaign? Democrats made it an important issue during Biden's administration, and even nominated a transgender secretary of health.
Yes and they celebrated him as the "first female four-star officer of the USPHSCC" despite the fact that he is male. As if this is somehow an achievement for women.