Two reasons:
1. The posts I'd find most interesting in the site are ones related to astronomy and applied machine learning. If there wasn't anything on astronomy (or physics) then I probably wouldn't use the site in the long run.
2.HN is a news aggregator. Most links that reach the front page have something interesting to offer. If I can't access the part of the site I find interesting, then I'll like go back to HN and find another post. (This isn't something I'd love to admit, but oh well.)