Well this Hub has been here for a while now, so the comments just stay and build up. Because it is a 'help' Hub designed for the Hubber's in the Community rather than the general public, usually it's the Hubbers who comment. If it's a Hub designed for the Public, generally there aren't a lot of comments.
I've noticed the non-writers who read, don't seem to feel comfortable making comments. The non-writers generally don't vote it up either. At one time I wasn't a member of HubPages and I read but didn't comment or vote. I thought it was for the 'members only'..
So to answer your question - Time. It just takes time.
Hope this helps!