A big part of it is people are just angry and stressed in general because the system we live in is fundamentally broken (pretty much no matter where you are in the world, though I am speaking through an American lens since the majority of Reddit is American).
Everyone can feel the effects of an economy and government that just doesn’t work for them. We’re fundamentally divided on how to fix it. Minorites are directly under attack and that manages to leach into most conversations, either directly or sideways. It makes people incredibly defensive.
The fediverse has a higher barrier to entry and, statistically, tech-minded people skew liberal. We’re a self-selecting community that is just more likely to agree – for instance – that trans people are people.
Further, since these services are decentralizedv and self-hosted, we can literally make hate groups unwelcome/banned from our instances because there is no profit motivation for hand-wringing like there is with Reddit.
Agreed, I grew up in a very conservative area and was pretty homophobic when I started college.
“They can do whatever they want, just don’t ask me to like it” was an important stepping stone towards “oh shit, love is love” and finally actually listening to the experience of gay people.