Pull-requests and forks are only a piece here.

It's not the only one implemented by tools that extend Git (Phabricator and Gerrit, for instance, support pre-commit workflow).

Github also brings a bug tracker (albeit a simple one), discoverability, hosting (obviously) and integration with many useful tools (CI, documentation generators, etc). All of this is easy to use.

These pieces greatly contribute to the success of many open-source projects (of various sizes). Github "extinguishes" alternative because of this, not because it designed an alternative to email-based workflows.

I suppose that many projects (open-source or not) want to use tools that extends Git. Maybe what we need is a standard making these tools interoperable and open.