Ask HN: What do you think of all these Hackernews clones?

In the last week, I saw a couple of HN clones that were subject specific (i.e HN for data science, HN for design...).

I am wondering what would be the motivation for creating a topic specific HN ? If it is useful why isn't HN itself creating subtopics ?

Personally, one reason I love HN is the serendipitous nature of discovering interesting stuff.