GitHub and GitLab are more polished and support different workflows, but the Gerrit workflow is really quite specific and is mostly unsupported by the regular GitHub/Lab tooling.
I moved from a company using GitHub (and a decade of GitHub based open source experience) to a company using a Gerrit-like workflow, and I can see why people would be resistant to a move. On the surface the options look quite similar, but "at scale" I can see Gerrit working quite well and missing certain equivalents that would make a transition hard.
That's not to say GitHub/Lab aren't better, or that a move might not be worth it for other reasons, but a transition that pushes people out of their comfort zone and reduces their productivity for unproven, potential, future gains, is a hard one to justify.
