* GitLab (and GitHub etc.) are organized around code repos but the hierarchy you use to organize your code isn't necessarily the best hierarchy to organize your bugs. There are a few different "plasma" repositories but end users aren't going to know which one the bug about their panel spazzing out on leap seconds goes in. So you consolidate everything into one Plasma project. But then your code and bugs aren't living together anymore, and using GitLab for the bugs buys you very little over just having good integration with Bugzilla.
* It's a lot easier to move bugs around between projects and they keep the same identifying number.