The question is very simple, why the Yearling badge is not available in the Area51?
However, it is still available in the Area51 Discussion zone. I was thinking that badge can be generic for all SE sites.
Area 51 is not a knowledge-building Q&A site where you're necessarily supposed to aspire to become an avid "expert site builder." It wasn't a behavior we were striving for. Area 51 was supposed to be about groups of experts in a specific field getting together to build a site.
So, long story short, much of the functionality was retained to make this process work, but much of it was ripped out where it simply did not make sense.