A good question. I suppose G&FH could piggy-back on the other popular pursuits…
"Click here to add BMD certificates to your Amazon basket"
"Hot, hot, HOT great-great-great-grandmothers GONE WILD!!"
That should bring in users, and revenue!
Failing that, G&FH.SE is neither well known nor readily findable. If I type "genealogy and family history" into Google UK (without quotes), I don't find this site until the bottom of page 12. (With quotes it's on page 2, which is better, but only for that particular query.) Family Tree Magazine's 25 Best Genealogy Websites for Beginners doesn't include it, and neither does GenealogyInTime magazine's Top 100 Genealogy Websites of 2015, as two examples of site lists.
A search of "link:genealogy.stackexchange.com/" turns up just six results, all of which are internal to Stack Exchange (@PolyGeo wins the cookie for top spot!). Nobody is linking here, so nobody is getting sent here. So it's very much a niche website.
SE itself is a pretty technical site, which isn't surprising given its history, and that can be off-putting. There's little explanation or hand-holding within the site to help new users or new genealogists. That SE even has a genealogy site isn't obvious from landing on the front page, so only explorers will get here. I'd be interested to know how many G&FH users came directly to this section, and how many were on SE for a different reason and found the genealogy section later. I was one of the latter, and I suspect a lot of others are too.
This is probably why G&FH has such a good signal:noise - many of the users are already somewhat prepared for rigorous questions and are ready to do their own research first. There aren't any of the "Seeking Joneses in Wisconsin, I'm sure we're related" type questions that litter Ancestry et al. But beginning genealogists just aren't going to find G&FH, or are going to be put off by the rejection of poorly researched posts. It seems to be more "experienced amateur" users who prosper here, and that's a much smaller audience. Quality is already present, it's quantity that's lacking. (How to maintain the former in the face of the latter is a different problem, of course. And "orientation" of many new users to the SE way will require effort.)
If G&FH is going to grow, it's going to have to become findable, and reach out "downwards" to less experienced enthusiasts. As @lkessler notes, unless word gets out, nobody will come. And there may be a chicken-and-egg issue here, in that G&FH is too small to be linked to by blogs, list sites and search engines, so it won't grow… But I think that finding ways to get G&FH mentioned elsewhere is key, even if it's as a "mature" Q&A site for the roadblocks and dead ends that everyone reaches eventually.