Apple bought them, took it down from the web, then quietly open-sourced it a few years later. They tried to make it popular, ran a conference for it, but the adoption was too minor for Apple to care afterwards.

It's still maintained by a sizable team at Apple, GH stats show that the activity is much lower now than it was 3 years ago, but there're about 10 people that contribute on a steady regular basis, which is honestly better than 99% of open source projects out there.