I think it's a mixture of dislike for Google/Chrome and the constant Chromium reskins that dominate the browser market + Firefox evangelism.
I'll personally say I don't hate this project as such, I mainly use a Chromium based browser because I build a product that uses the FileSystem API with a directory picker [0] and the FileSystem Observer API [1]. Both of which aren't supported anywhere except Chromium based browsers.
I used to use Thorium, but last build was out in February and I simply can't use that as my main browser, so now I simply use ungoogled-chromium.
I can easily avoid any Google telemetry, use uBlock Origin with MV2, Privacy Badger and the plus point is all sites work. I love Firefox as well, but I feel you can't do much about the fact all "regular" people end up using Chrome or Safari; so while developing on the web you simply can't become a Firefox main and avoid chromium entirely.
Generally because there's not that much to comment on, as this is effectively a somewhat prettier ungoogled chromium, and so there's not much room to make positive comments. Though I agree people get caught up in negativity easily.
