Because the crazy liberal woke crowd was brought in by queer theory and non-transmed trans advocates. The lumping is not a right-wing idea at all. If you're stuck in the middle of "LGBTQ+" for solidarity (as a portion of the T), when LGBTQ+ draws the ire of some group, trans people face ire in solidarity.
And queer theory is really good at challenging the dogma of gender and sexuality, which of course makes it anathema to conservatives who "preserve traditions" rather than "challenge dogma".
For example, queer theory is, among other things, about "interpreting identity as a socially constructed phenomenon and sexuality as being fluid in order to expose the ways gender roles and stereotypes are reinforced by notions of identity and sexuality as being fixed". (Summary from Wikipedia.) One could think this is a good thing, or not, or be uncertain...and any of those stances are compatible with treating people with gender dysphoria.
I feel really bad for people who have gender dysphoria and are being used as pawns in a culture war. I wish we could just support people who most need it while recognizing that there are tradeoffs and not everything is possible for everyone. It seems like accommodating people with serious gender dysphoria ought to be a high priority. You don't need to challenge most of the assumptions of cisnormativity to do that.
But instead of being focused on treating gender dysphoria, there's vicious rhetoric between nominal allies who leap to use gender dysphoria (or intersex conditions) when challenged but then propose policies and make statements about trans identity that only make sense if gender dysphoria isn't really what they care about; and enemies who use being anti-trans to rally the faithful: they take the very worst of the previous arguments and point out rightly that the particular argument makes very little sense, and then conclude completely wrongly that being trans is nothing more than perverse and destructive ideology.
For instance, Matt Walsh's "what is a woman" video is as effective as it is (which is to say, moderately--which is only news because almost everything else he's done is transparently awful) because when he interviews people they're tripping all over themselves to make sure they give a fully inclusive definition, not just one that covers people with gender dysphoria.
If you only need to include cis women and trans women with gender dysphoria, you just say: "A woman is someone who is, or has a deep-seated visceral sense of being, an adult human female. Cis women are physiologically female; trans women have gender dysphoria because they mentally are 'meant to be female' but their body's development didn't match, which causes them distress."
This is easy.
But you can't say just that or you leave out non-gender-dysphoric trans people.
For a longer and better explanation of why the dysphoria-centered view of trans rights is rhetorically powerful (i.e. convincing) where other approaches are not, read TaraElla's perspective: https://medium.com/trans-sandwiched/why-gender-dysphoria-rather-than-gender-identity-explains-transition-19d5a65e6311
If it had been intentional--I suspect it mostly wasn't--I would think that the gamble had been that by sacrificing gender dysphoria as a defining characteristic and only occasionally using it in arguments, the LGBTQ+ community would be able to win greater acceptance for every type of non-cis-heteronormativity. But in a lot of the U.S. instead of winning greater rights for everyone, they've failed to do so and have lost the rights of people that otherwise would have had them. No credit to the real villains of the story--the reactionary right who is happy to inflict suffering on people who are different for political gain and religious ideology, even if some of them actually know that there's a distinction there that they would have to support if confronted. But to me it looks like the gender dysphoric trans crowd has basically been used as a human shield, and the repressive right is firing right through the shield.
I really don't know what to do at this point aside from getting trans people out of hostile states in the U.S. (it seems crazy, but when legislatures are banning all gender affirming care for everyone, and the right has a supermajority in the state legislature, what else is there to do?), and try not to end up running the same strategy to the bitter end in the U.K., lest it go too close to the same way.