Because any price at all will immediately cause users to shift to another platform, and GitHub's value is that it is _the_ place to put your code on the internet.
There's a lot of consumer surplus with the features GitHub offers. I'm sure they can have better pricing tiers to capture the surplus without losing too many users.
I wonder how much network effects help here.
There's a lot of consumer surplus with the features GitHub offers. I'm sure they can have better pricing tiers to capture the surplus without losing too many users.