My reasons are many.
I've been dilligent for years on having a better social media experience. Rejecting algorithmic hellscapes, AI horrors beyond our comprehension, and artisinal time sinks, carefully crafted in the Silicon Valley. I stuck to two places primarily; fedi and Discord. The former because I like it, the latter because it's where my friends are, and the alternatives suck.
All of that hard work has been helpful, but I still have the seeds of social media addiction in me. I'm obsessive over unread messages, engage in arguments that neither party wants to have, and I don't like the kind of person I've become when it comes to how I talk to people. Leaving those spaces entirely is unhelpful for its own reasons, (todo: there's a good blog post on the web about this, I should link to it) but they still burden me immensely.
I'm always going to be "too online", and I don't have access to good offline alternatives. I need something to occupy myself with that isn't social media. And well, the idea behind Gemini is that it's just social enough I feel like my thoughts are worth talking about, even if no one reads them.
The old web nostalgia can be really harmful and hostile, especially to people like me, but I do like the simplicity behind things that I can understand and keep all the important aspects in my head. I hate modern "conviniences" like smart TVs and IOT and awful frameworks for building bloated software that barely work. Having something I can point to and go "Let's do this" is helpful, so I take inspiration from a rose tinted perspective on how things used to be.
I'm too connected to people, and also too distant. I miss going outside and seeing friends, doing dumb shit together and having a brief pause from all the stress in the world. I miss the brief glimpse of raw freedom, and maybe I can capture that magic in a computer, if I just do it right. (I know it's futile, but funny projects give me hope long enough for the next funny project to find me)