This is not about licensing. The problem is that DocC is incompatible with the current tools that many Open Source projects use. Sure, you could use DocC for

This is not about licensing. The problem is that DocC is incompatible with the current tools that many Open Source projects use. Sure, you could use DocC for an OS project and set up your own server to host it. It's probably pretty cheap to do that on a VM. It would require just a little bit of learning about web hosting and would require minimal maintenance. But why would anyone do that when Github pages is completely free, requires zero maintenance, and everyone knows how to use it already? A lot of small Open Source projects currently use Github + Github pages for hosting docs because it is such a convenient offering. If you want to use DocC, you have to look for something else.