Why do FOSS projects use github? Because it's a good product, simple. It has lots of features that people use every day that you don't necessarily find on other source code platforms. For example, their new code search is top class. You won't realize what's missing until you try to do the same thing elsewhere. GitHub also gets a lot of details right e.g. in pull requests and issue management.
Added to that, it has the largest community of developers, and those who would potentially participate in FOSS projects likely already have an account. It's where people already are and what they are familiar with.
(I have contributed to a few FOSS projects on github.)
This ‘everyone is using it so I need to also’ logic is funny to me, given the idea of the F in FOSS is to use network effects to spread the idea of software freedom. Using and thus endorsing a proprietary platform with ever increasing integration into the software lifecycle seems to do the exact opposite of that.
The code search is rather meh than top class. Any grep can do better. Also the code search requires one to log in, basically does not exist for not logged in users, which equals zero quality for them.
Added to that, it has the largest community of developers, and those who would potentially participate in FOSS projects likely already have an account. It's where people already are and what they are familiar with.
(I have contributed to a few FOSS projects on github.)