[richardofyork]

A few reasons:

1. People like the sense of security that comes with a paid (and perceived well managed) service. Indeed, GitHub is well managed and the streamlined process makes it easy to use.

2. It has become both the cool and the expected thing for developers to use. It has just about reached de facto status.

3. It is well maintained by a very professional, known staff.

4. While it is not completely free (it has a limited free account), it is not prohibitively expensive. And developers have a few extra dollars to pay for this service that actually helps their careers and their profiles.

5. It has become social to the extent that it is the place for developers to be.