If you look at the contribution activity it dropped off after acquisition of GitHub by Microsoft and it seems that development was redirected to VSCode.

The writing was on the wall for a long while now, and was one of the reasons why the JuliaCommunity stopped advocating Juno/Atom as a platform and instead switched to VSCode

Probably a bit of both if I was to hazard a guess. MS has VS Code, which is definitely the elephant in the room as far as go to IDEs in the environments I’ve been in lately. So right now they’re spending engineering time on both VS Code and Atom, so they may as well focus their resources on the more popular one.