The less succinct version is that she was considered one of the OG progressive youtubers who gained a huge following and as a result has garnered a LOT of harassment over the years she was running. (Keep in mind she was at peak of her youtube career around the time of "Gamergate") When she directed mild criticism towards the Disney movie "Raya and the Last Dragon" and how it was derivative of Avatar, I can't remember the details but something about the way she made the comparison set her up to be attacked by online chuds pretending to be concerned progressives attacking her for being "anti-asian."

Lindsay Ellis was a champion of equality and diversity and the details were infuriatingly minor and stupid (which is why I can't even remember it) but it was just the final straw and her mental health couldn't handle any more threats, attacks and condemnations and she left Youtube other than doing some guest bits with other old-time Breadtubers and she still does work on Nebula.

I fully get how even if you make a widely praised and well-received work, you will tend to fixate on the negative reviews the most, but this was the end of a long-term campaign to bully her off the internet and we can't expect every content creator to have inhuman thick skin. That said, I wish she would have just got off Twitter and kept making content, we could use her more than ever.

I think people got mad at something she said about Avatar on twitter (the cartoon not the movie), and her anti-fandom reached a critical mass spreading around every single thing she'd ever said that could be interpreted uncharitably, and with her book out and her presence on Nebula secured she decided that being on YouTube was more trouble than its worth.