It is fairly simple: having a GitHub account and being active on StackOverflow don't appeal to the majority of developers. Those sites will capture far less than 1% (made up, but probably close) of developers making them not the best location for hiring managers to look.

IMO, if you want to find developers to hire then LinkedIn and recommendations from other developers are still the best route. Having a successful GitHub project doesn't always translate to "great team developer" as different skills are required for each. Being helpful on StackOverflow for a very specific thread doesn't always mean you are a master of your craft. Having both experience and recommendations from the industry and category being hired for is more of a "guarantee".

Disclaimer: I use both GitHub and SO.