J2O and the JCF are not as popular as they could be because people don't know about them. If you do a quick search, you'll find lots of topics where someone mentions Jazz Jackrabbit - usually in the context of "anyone remember" - and lots of people express their nostalgia, assuming Jazz is dead. We - or the majority of us, although I can count myself among those who migrated from the JMMB - got here because we care enough about Jazz to find J2O. But there are more people out there who have never heard of J2O. We need to put more effort into advertising. I try, but I can only do so much - Baggers is one example.
I was talking with Fquist about this a while back, and we never actually got anything done, but one suggestion is to advertise Jazz and J2O wherever you go. My standard signature consists of links to J2O, Haze's Hideout (demo section) and JCF, more or less depending on the maximum lengths of the forums. My standard avatar is this:

And I use it everywhere except obviously here. These are not hard to do, but they can make a difference, and I know I've caused at least one person to buy JJ2 and investigate the community.
In summary, you can't just expect J2O. JCF and Jazz to become more popular on their own. Go out and find people and change the world.