Because jj is vastly simpler and more powerful than git, while being compatible with git (so you can keep using Github etc). There's tons of articles and videos on the topic, and many other posts here on HN about it

Check out jjui in particular. Makes it even easier

Git makes certain workflows more difficult than they could be. Using a different tool that makes you feel more productive is generally nice.

git has a terrible user experience, but you use it because everyone else is and it's powerful — much like Linux

it would be good if there was a powerful VCS with a good user experience and git compatibility