It's a power user tool that has a slight learning curve.
GitHub and others are frequently code browsing tools first, with code review coming second. "Code hosting has look like GitHub" is unfortunately a common myopia.
Phabricator (from the little I used it) seems closer to Gerrit than other code hosting.
If you're used to the GitHub pull request style, you'd be more likely to make a small commit and add it to the request. In gerrit that approach results in a separate patch for review, which isn't what such a person would want.
(Personally, I've come to appreciate gerrit's model for strongly encouraging building patches that are encapsulated single-reviewable-changes, in a way that lends itself to never having a commit that'd break the build if you landed on it e.g. during a git bisect.... But it's also something that surprises people who've not used gerrit before, in my experience.)
It started that way for sure, but we’ve evolved with boards, milestones, epics, swim lanes, burn down charts and a lot more.
We have a whole group in our engineering and product org focused on making it even better: https://about.gitlab.com/direction/plan/
Even on installing the self hosted Gitlab, the open source version is hidden, with the free yet closed source version being pushed.