Probably Atlassian was sick of people never upgrading their old installations and getting hacked for it, and people did not upgrade because it is quite a hassle in the first place, not to mention plugins breaking allll the time.
Oh and because cloud forces continuous payment whereas prior many customers simply bought a one year license and went on without renewing support.
We're running a selfhosted Gitlab Premium since 2019. The only two times in the last 3 years we had issues with artifacts not being deleted (causing nightly backups to become 500gb, will be fixed in the next version) and some out of date apt certs to run "apt update". Otherwise, I update Gitlab every month without problems.
Gitlab is a breeze to upgrade when using the Docker distribution. Swap the version number in Kubernetes or the systemd unit file (if you're using naked Docker), restart the service, that's it...
Atlassian's docker images are similarly easy to use, but with everything Atlassian you have a veritable ecosystem of plugins of which almost none are open source so you are out of luck if there are incompatibilities.
Oh and because cloud forces continuous payment whereas prior many customers simply bought a one year license and went on without renewing support.