Not all projects have the resources to self host GitLab CE. Actually compute resources may not be much, but keeping it up to date with security patches can be burdensome.
Dunno why you got downvoted. Some FOSS projects have zero server resources, and GitLab updates are... well... let's just say I had to turn off automatic updates for our instance at work because of how frequently they broke it, meaning I have to manually update it every few weeks.