It's a single, unified place to host code with a known-to-be-good experience.

hzy has it right on the money. It's rock solid and reliable. The built in issues/pull requests/wiki are fantastic. Adding in a collaborator is easy. It's credentials I already heavily use.

Plus, I like github tons so I'm happy to pay.

One of the most compelling feature of using GitHub for private repositories is developers already know how to use it, already have accounts etc. GitHub has a nice growing moat in that regard.