Unfortunately for them. The market is already saturated with CI tools, including good ones.
- If you want good self-hosted CI, you use teamcity (jetbrains) or bamboo (atlassian). Side note: They cost money, you get what you pay for.
- If you want good SaaS CI, you use travis-ci (linux), circle-ci (linux) or appveyor (windows).
- If you want to suffer endlessly, you use Jenkins (previously hudson). It's shit, it has a Bad UI, it's an aggregation of poorly maintained plugins, it lack even the most simple features, the list goes on...
- If you want to go exotic, you can find dozens of other [partial] CI tools.
There is no room for gitlab. Teamcity already has a free edition offering 20 projects and 3 slaves. All the aforementioned tools are free for open-source projects.
Disclaimer: I have used all the tools mentioned above.
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The reason the good tools are not popular is:
1) they cost money and people are bitches when it comes to spending even $10
2) most people start with the old well-known shitty tools and then they're locked in... and the efforts required to move away just increase over time (sadly, nobody got fired for choosing Jenkins in the first place :( )