My guess is the network-effect. If you want more contributions Github is the platform to pick for your project. Good or bad it is the de facto standard platform for OSS projects with a huge pool of potential developers.
I agree but think it's a bit of both and depends on the project. I e.g. don't think SDL needs more exposure. Everyone in game dev knows SDL.. many game engines / frameworks are built on top of it.
That's it. Back in the day that's what was great about source forge. Now it's github. Many people already have an account there so you significantly lower the bar for people to get involved with patches, bug reports etc.
If a project has self hosted infra and wants me to sign up to their Bugzilla I think twice before doing so.
And if github really turns to shit because Microsoft shows their true evil face you can just move platforms again.
Getting new contributors is overrated for OSS unless they are an exceptional contributor (like if a big org were to contribute).
Main incentive is getting more consumers of the project (increase popularity) and making it more convenient for current team.