First of all, I Love your forum avatar! That cat is Darling!
I think there are two parts to your question - why is it not active and secondly how do we attract more people.
I think the answer to the first is that these things go in waves. And also, for many people these topics, the same topics, have been covered in a the past, so maybe not everybody responds to a thread if they've participated in a similar one already. Which isn't helpful to newer members, and it's true, makes the forum look less vital. I think, as forums go, this one has a fair number of readers. One thing that I note is that more people post in the market section advertising cats than actually post in the discussion section. I think a lot of people in the community are active and may peruse but not be chatty here, for whatever reason.
For the second part, I think that the most easy people to engage are the people who already visit the form as a habit, in the market section, are the ones most easy to bring in. I think it's an interesting topic and maybe there could be more outreach inworld. I totally agree that a lively discussion section is a good thing. Kayla's thread on Murphy's Law is an example of a fun sort of thread that adds. Maybe more Showroom threads with Themes of kitties, where people put up pictures of their cats. I always love looking at those. Winter's pics have fun stories to them, Kayla's, Maxwell Grantly hasn't done his fabulous picture stories in awhile but those were Great!. John's comics.
This may also just represent a lull. At the risk of howling from the audience, I will mention that SL2 is coming up, and when I mentioned that in the context of another thread people took the thread into derail (my first post about SL2 was Not a derail, but very much pertained to SL1). However, SL2 and other virtual worlds ARE coming, and the more information we have about that, the more solid things we will have to discuss about that.
That other thread, which was about permapet costs, is an example I think of what you're talking about. It was an old thread, 6 pages long when someone brought it up again, and after the derail there wasn't much posting, and I really think the reason for that is that many people had commented on the topic before and didn't feel the need to comment again. It wasn't that they don't care about the topic.