Marketshare.

Everyone knows about GitHub because everyone shares links to their projects hosted on GitHub! Same reason they provide https://gist.github.com/ and http://pages.github.com

Whether you eventually want to start your own business or just want to work for a good company ; you still end up at the same place - in a team environment, needing secure, painless version control hosting. I can tell you right now there is no other service I would use or recommend other than GitHub.

The same question could be asked why Github gives away so many free repos to open-source projects... :)

Bitbucket is a business too. I have no issue paying them.

Besides, storing bytes costs pennies, or less.