If reddit became NSFW with ads you'd see a huge drop in traffic from people at work (that don't have access to Adblock on their computers) and a lot more people not disabling it for reddit.com at home.
Not only that, but if reddit started carrying those ads appropriate to its demographic, it would face a massive revolt from its own users. Redditors get irrationally angry at reddit whenever an ad targeted at them so much as plays sound, let alone insults their self-image as high-brow intellectuals who “deserve” premium ads on par with the sponsors of Condé Nast’s other properties. (Pretty funny stuff, for a site that had to petition Google to hide “jailbait” and “nsfw” from its top eight sitelinks.) But yes, that’s the dilemma.