one word, ADS.
If you're suggesting adding ads to the forum, there's no reason that would achieve anything. It would mean the site pays for itself to some extent, at least if we assume people don't use adblockers (I suspect most visitors here are very much the kind of people who know how to, and probably do, use an adblocker, myself included) or whitelist this site (I suspect this would be 50/50 at best). People aren't going to say "oh, this site has ads, I'll post on it more!". Either it puts them off, or they don't care either way.
If you're suggesting advertising the site elsewhere - casual mentions on related sites, sure, but not much is going to be achieved by buying random Google Ad space and saying "hey come to Lemmings Forums". This is the kind of site that, if people are interested in,
they'll find it. People advertise because they get a return on their investment - they might pay $50 for an ad slot, if the ad gets the attention of 100 people who each spend $5 on that site, the site has made a profit of $450. We're not selling anything; we don't make money from people coming to the site (and nor would I want to; I'll gladly accept donations to help cover the costs of running the site, but that's it, the site is not about making profit and never will be). It would be pointless for us to therefore spend money on ad space to try and get people to come.