The target audience for this is college kids, who these days frequently use GitHub for consuming course material (e.g. boilerplate for class assignments). Doesn't seem like a bad move on their part to avoid rolling their own account system. People like one-click auth.

Yeah, but they may not have an account on github.

And I see more and more students using gitlab instead since they have free private repos

Students get free private repos on Github too with the education pack.

Oh it’s free, that’s all right then.

Many people don’t use cost as the sole measure of everything.

Sure, but the parent commenter explicitly mentioned free as a reason for using GitLab.

So then they can make a GitHub account... I don't see how that would be worse than having to make a custom HackMIT account.