I recall we considered this at various stages a few years back. I think this was around the same time we introduced our chatops integration.

I don't know the exact details and can't speak for others, but I suspect GitLab realised that chat(ops) tools are hard to sell and our time is better spent elsewhere. It's a shame though, having Gitter/chat directly integrated in a project (properly, not using e.g. an iframe) would've been pretty cool.