Why Github? Copying from one commercial provider to another doesn't solve the fundamental problem. Using git helps, but most of those old repos will never get cloned. In 10 years time, Github may be the tired old service that gets acquired by a hedge fund that decides to monetize their repos. Such things are part of the corporate lifecycle.
For now, Github is not ad-ridden as SourceForge is. Github is monetizing some repositories: https://github.com/pricing I don't know if they're sustainable, but from my naive point of view, closed-source projects on github pay for the hosting of open-source projects on github.
My company uses GitHub Enterprise. Unless we have some sort of deal/discount above the built-in, we're paying over $30,000/year for it and we run it on our own servers. I'm guessing a lot of other companies do as well. Developers are quite used to using both git and GitHub and $30,000 is nothing if you have a hundred developers costing you $150k a piece (not just salary, but computers, benefits, desks/office space, payroll taxes, etc).
SorceForge counted on their open-source stance limiting who would use their service and, by extension, limiting the resources they would need to serve those people. GitHub works the opposite way. They want everyone to think of GitHub as "the place I put stuff". Have a code snippet? Stick it on GitHub! Want a basic wiki for something vaguely code related? Create a GitHub repo just for the wiki! Collaborating with friends on a class project? GitHub! And then, years later, GitHub feels like second-nature to you and you love it when employers are using it paying GitHub tens of thousands a year for it.
I'm not accusing GitHub of doing something nefarious to lock people into GitHub. Just noting that GitHub feels very familiar and that makes GitHub a very reasonable choice for companies who pay them money. Without that familiarity, the value of GitHub isn't the same. If you're a company spending millions per year, $30k is a drop in the bucket for software your developers are already familiar with and software that works well, is well supported, and can handle your problem.
Yes, GitLab exists and has both open-source and enterprise versions, but I'm not sure that a business feels that differently about $5,000/year for a 100 person team and $25,000. I'm glad GitLab exists, I'm glad Bitbucket exists. They'll make sure that GitHub has to continue being great and they'll provide services to people that want something a bit different. But GitHub's business model seems pretty sound. The more people use GitHub for free, the more likely high-rollers are to pay for GitHub.
I mean, the GitHub subscription per developer costs less than the additional money my company pays for Apple gear for developers. By targeting open source with a premium, free, non-ad driven product, GitHub opened the door to lucrative business sales. They seem like a sustainable business and it even seems like the free, open-source repositories are part of that business plan.
I'm not saying that Apple gear is so overpriced or that it isn't a better platform to develop on, but we don't need retina displays to do our work. And many people argue that you don't want to force devs to work on a platform they're less productive on. The same applies to GitHub. If your devs are more productive or, heck, even happier or more comfortable using it, $250/year isn't something a company is going to blink at if it's paying $150k+ per dev - just as the company won't mind paying an extra $100, $500, or $1,000 in equipment for that dev.