First, because only the https://github.com/popcorn-team/popcorn-app has been removed. There are plenty of forks of the original popcorn floating around github, some of them with very recent commits (see the network timeline). It seems unlikely that github would remove just that one repo and leave all these others around.
Second, because as noted by some commenters, the website of that fork http://popcorn.cdnjd.com/ is also down. But it is not just down: the whole A/CNAME record does not exist anymore! Digging a bit deeper, you can see that the nameservers of that domain are hosted by Cloudfare.
It seems very unlikely to me that Github and Cloudfare would act so well coordinated in taking down an application and its associated resources, yet they completely ignore all those forks.
That makes me think that this is more of a personal issue. Either someone with all the keys got hacked and that's the result, or someone has been intimidated enough (through legal or other means) to shut everything down.
Anyway, time will tell. Don't start migrating all your github repos elsewhere yet ;P