I've no idea why it hasn't gained more traction, but here are my own reasons for liking it enough to spend more time with it nowadays:
- It's improved quite a bit over the years, in terms of being user friendly and feature rich. And it looks better.
- The previous hubs that I was on were painfully slow. Now it seems faster, even on my shared web hosting arrangement.
- I lost all my data a couple of times because I didn't take care to clone or backup my channels sufficiently.

Additionally:

It doesn't get enough good press, and sometimes what is said about it isn't positive. For example, the indieweb.org site says: "Very confusing UI / UX... Finding how to post to the channel is non-obvious."

(I would argue that it's a hell of a lot easier to implement the principles of the indieweb through Hubzilla than some of the proposals on the indieweb site.) Hubzilla is actually less complex than, say, Facebook, but people use what they are used to.