Just the latest social group that's still broadly acceptable to shit on.
There's not a ton of global census data out there, but in Canada trans and non-binary people make up 0.33% of the population. Which means there's a lot of people who don't know anyone who is trans or non-binary. Unfortunately there's also a lot of people who are unwilling to emphasize, or even sympathize, for those they feel are different or strange to them. It take time and effort to listen to others' stories and to gain appreciation for their perspective, and it's an effort many people are uncomfortable making if it feels they are deviating too far from society's norm. What you're observing is those in power taking advantage of the same human weakness that's been used forever to discriminate on whoever the current permissible outgroup to hate is.
How many times have you heard, "I don't care about anyone being/doing Y, but...", and then proceed to say some sort of transphobic, homophobic, racist, or sexist shit? When I grew up it was the G in LGBT. When my parents grew up it was African Americans. Women only got the right to vote a century ago, you better believe some of our great granduncles had some shit to say that would make today's uncles look like saints.