Because men with fetishes aren’t social rejects, they’re the standard and are some of the richest and most influential men in the world. They quite literally have conspiracy level connections and resources to enact whatever agenda they want.

The thing is, as well-meaning as the left-wing is, nobodies' politics are pure, and everyone wants the safety net of being seen to be a good person without actually being a good person. Being a good person - being actively considerate at most if not all times - is hard. And even more hard is having good politics, because it requires deep thought, careful consideration, constant reflection, and a decent chunk of humility - and, at its core, it requires some sort of desire for the destruction of hierarchies and the reconstruction of society. The left-wing has a lot of oppressors in it - white people, straight people, men - who ostensibly appreciate the mainstreaming of good moral politics, but ultimately do not wish to sacrifice the privileges they have.

So, then: we have a tension between what is required of us, and what we subconsciously want. Trans politics got mainstreamed fast because it's an axis of oppression that everyone can easily claim to recognise precisely because it doesn't exist. It costs zero reflection to say 'he, she or they' - and bonus points!! it actually evades any recognition of, and reflection on, axes of oppression that actually do exist: sex, sexuality, even mental health and intersex conditions. Given how misogyny is the oldest, most widespread and most deeply conditioned oppression in human history, feminism - even the most milquetoast, sex-positive brand of feminism - was causing major cultural tensions. We had the first wave of backlash with gamergate and 'sexist air conditioning', and feminism has majorly backtracked since then, embarrassed and desperately seeking to curry favour with the oppressor class. Trans activism has been championed by mainstream feminism as an apology, and it's entered mainstream culture by storm for the exact same reason.

Basically the traditional conservative position, the thesis if you will, was "gender is real, gender roles are innate, and you have to follow the rules", and the traditional liberal view, or the antithesis, which was "rules are bullshit, I shouldn't be expected to have to be a certain way because I happen to have been born a certain gender." By divorcing the idea of gender from biological reality, liberals got to form the synthesis of these two ideas. That way they can have their cake and eat it too by not being bound by traditional gender roles but also still getting to reinforce them. Because instead of freeing themselves from gender roles they now see liberation as bouncing from one arbitrary gender role to another.

The fight for gay marriage was (alongside abortion rights) one of the most critical single issues mobilizing the left as a voting bloc. In politics, both the left and right have specific highly-divisive policy issues that are meant to drive voters into the ballot box in support of one side or the other. Generally, the importance of issues like these also compel voters to support policies and individuals they may not otherwise support or agree with (e.g. “i don’t agree with Sen. X about economics, but at least they support Y!”).

Gay rights and abortion are the two big ones for the left. So what happens when one of the issues is resolved and a vacuum appears? People who have fought long and hard for equality will look for the next issue they can help lend their experience to; organizations and politicians will continue looking to mobilize volunteers, donors, and voters to mobilize around another issue that has as much influence and power as the fight for gay marriage. It is literally an existential question because without the LGBT+Ally voting bloc (especially the allies who want to feel good about their voting choices), Democrats and the left don’t have the same kind of leverage against other policy issues.

There’s actually an argument to be made here about if this partly explains why Democrats in power were not able or willing to pass full nationally-legalized abortion rights in the decades before Roe v. Wade was finally overturned—once abortion stops being a divisive policy issues, it stops driving donors, volunteers, and voters to the Left over that one issue.

So in the vacuum of Legal Gay Marriage grew another policy linked to a marginalized group of people under a narrative that read just like gay and civil rights: people rejected for who they were by conservatives and bigots, people attacked senselessly for what was beyond their control, people who deserved respect, dignity, and recognition. Defending trans rights became the next big issue. A narrative that I myself believed was, “why would a man (privileged) choose to be a woman (marginalized), and go though the social and physical difficulties of coming out as a woman, unless that was truly their identity? Don’t they deserve protection, as perhaps the most marginalized [gender] class of people, because they clearly had no choice?” It wasn’t until later, when i was still grappling with my own gender dysphoria, that it became increasingly obvious how sexism remains intact through transitioning, and how sexism (and fetishization) is in fact the foundation of the trans rights movement. And it’s a testament to this sexism that nearly every trans rights issue is directly counter to women’s rights issues, and uses the values of feminism to degrade (sex-based) protections for women, girls, and all female people.

And politically, trans rights is big money because it promotes consumerism as identity (as Dr. Based says above) AND it supports the pharmaceutical industry via lifelong medical care (hormones, plastic surgery, treatments following the damage caused by hormonal issues eg reduced bone density in females). It’s big money because there’s a black-and-white narrative that has been constructed to mirror every major civil rights issue of the past century: “we just want to exist, stop hurting us” which is VERY easy to take at face value as a political group (Left especially) who values harmony, equality, social justice, and diversity.

Unfortunately this also leads to male rapists placed in women’s prisons, willful ignorance toward how males (throughout ALL OF HISTORY) pose a unique and significant threat toward female people that has nothing to do with mindset or “identity” and everything to do with sex (+reproduction and reproductive control), the comparative social/economic/political capital of trans women over that of trans men, the pressure to deprioritize female issues or make everything “gender neutral” specifically for female health issues.

When Obergefell passed, every LGBT+ organization and major Democrat/Leftist platform in the US switched gears to trans rights, which was a smart economic move to continue holding onto power. Not only that, but the most politically advantageous fight is a fight that never stops. The goalposts will keep moving, the demands will get more extreme, and any backlash will be villainized to further drive supporters into echo chambers.

What’s also ironic (not) is how many Republican/right-wing individuals (and even countries like Iran) are being given credit for being “progressive” about trans issues, even when it’s because trans ideology actually supports a strict binary of gender performance: you “act like a” boy/girl, so that must mean that you are one.