Fair question.
Right now they are made purely from organizations that have:
- published Julia repos
- Julia in their name
After we got some more users, I wanted to get community feedback on how best to generate and populate categories.
Sorry if this is an inadequate answer 
Also, feel free to post an issue to get the ball rolling.
// edit: as @FemtoTrader pointed out, maybe topics are the way to solve this: pkg topics