Two reasons, IMO:
- I’m misusing Discourse as some kind of Wiki/Memo app (or blog, if you like) to document great code or links that I came across. Those kind of postings usually do not need/attract replies.
- I’m having too few users/readers to contribute something valuable.
All the things I put in my Discourse instance could have been put into my WordPress blog instead, too, but I love Discourse so much that I absolutely want to support it and use it by having my own instance of it.