As an industry, we have a unhealthy obsession with change for change's sake. If we aren't redesigning everything, adding new features, or moving cheese all the time, we aren't innovating.
But this the opposite of how we should think. If the GitHub repos page, for instance, just works, then we don't need to keep changing it. Zach Holman made a point, way back in 2012, that they intentionally hide UI features to preserve simplicity and trust in their design. [0]
The thing I get the most from Gitlab's UI is this overwhelming sense of desire to add every feature, expose every option, and make it as utilitarian as possible. In doing so, though, Gitlab trades off approachability. To some this is a great thing, but to others, it's just on the edge of too much. If there's anything I want Gitlab to copy from GitHub, it's the opinionated decision making of what to show and when to show it.
Gitlab's CI/CD stuff is a great example of this contrast. GitHub left it to Travis and other CI providers, which makes both GitHub and those providers have time to excel at what they're good at. They made the trade off deciding that they couldn't pull it off as well as others, so they delegated it and moved on. Gitlab took the opposite approach and built it in. While it adds value for some users, it can and (at least in my opinion) does over complicate the core feature set they offer.
It's okay to have two products with different approaches, but I wouldn't say that it's a lack of innovation on GitHub's part. I would say it's just a different set of tradeoff balancing.
[0]: https://speakerdeck.com/holman/git-and-github-secrets?slide=...