I think there are many factors and it's hard to pinpoint one single reason. Different people write different topics, and even for the same person, they can be writing about photography today and then Splinterlands tomorrow.
For me, I think it's a mixture of the stuff you listed. If I have to rank, probably the content has to be interesting and to my liking. After that, then it's the networking, the appreciation, etc.
!LUV