SQL (4.03%)
Lines changed on GitHub: 1,347
Tagged on StackOverflow: 363,595
It doesn't have that many lines on GH when you label it as SQL, but compare that to PL/SQL: PLSQL (0.16%)
Lines changed on GitHub: 3,561,189
Tagged on StackOverflow: 14,488
2. Their competitors (GitLab, Bitbucket) already have that offering.
3. I don't think they're interested in GitHub for its revenue, but for its strategic value. GitHub has to protect its revenue model by disallowing free private repos, Microsoft can afford to be more flexible.
4. They've already made similar moves. They acquired Xamarin, which was expensive, and made it free.