First of all, I'm surprised at the level of depth you get on the sector. That said, my comment is for show you a constructive opinion. Like an active IT guy who has used GitLab and migrated to GitHub I'll give you some numbers. I would talk about GitLab and GitHub, beacause I used it (bitbucket too, but 8 years ago).
Searching for public data on the usage, GitHub is the most used in terms of CI/CD: https://survey.stackoverflow.co/2022#section-version-control-version-control-platforms and https://www.jetbrains.com/es-es/lp/devecosystem-2022/data-science/ (peeked 2022 because in 2023 GitLab does not appear).
In a popular forum for IT, GitHub actions is a more active topic: https://stackoverflow.com/collectives/ci-cd?tab=tags
Google trends show that GitHub actions is by far most searched: https://trends.google.es/trends/explore?date=today%205-y&q=github%20actions,gitlab%20cicd&hl=es
The key for GitHub actions is that you can write your own, becoming part of the open source community. Or using any actions from anyone (or organizations) that they share and maintain.
* GH go too slow integrating the AI in the platform. And nowadays, some features like you say (test IA, for example) it's still in a WIP status.
* The AI stack integrating all the process (PR, Copilot for Codebase, Testpilot), it's more mature.
* You pointed it, the large orgs, can host for herself the platform. It's an important thing, when talking about law compliance.
* It's Open Source.
* Almost, the entire Open Source community is at GitHub. It's hard to change the people's mind and habits. And, if you want to contribute to the software/library, you need and account in GitHub.
* The pricing is an important factor, ~x5 more expensive.
* Visual Studio Code is a popular IDE used by a lot of developers. It's fully integrated with GitHub.
* Copilot is a game changer that become the most popular.
* GitHub discussions is a feature used by ~16%: https://survey.stackoverflow.co/2023/#section-most-popular-technologies-asynchronous-tools
* Microsoft have Azure as a cloud provider. He can integrate it with GitHub to make a smooth experience for developers.
My final words: an amazing deep dive that I really enjoyed. Not sure if GitLab can become a market leader, but they are doing good things. The time would say. For now, it's not the case. I think it's important to monitor the stackoverflow and jetbrains surveys, beacause it shows key insights from the sector =D.