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.
For more obscure topic, it's more difficult to remember all the names, but I recognize some of them when I saw. For example, I remember that there was a big plane accident, and the first comment was from a pilot that recommended a specialized forum with more technical information that the standard press coverage. I also remember a discussion about the Helium content in the universe and one of the comments was from one of the scientific that measured it a few years ago.
The motivation, I think, is primarily twofold - a desire to 'recreate' HN with a more limited focus, or with a different and more up-to-date UI which still maintains the minimalism people like about the original. Why make a 'HN for data science?' Maybe because stories about data science can't get the focus one might like here.
Serendipity has led me to find all sorts of wacky stuff here that I would never have found otherwise.
Perhaps they may be some sort of benefit for having a "HN for [insert topic here]" for people to find a narrower information set. Personally, I like it as it is and one can always use the search function to target posts with keywords