Because regardless of how right it may feel, it more often than not will ruin the integral structure of a family and destroy trust if all members of the family aren't on the same page.
Look at the Rob story for example: assuming it's real (There's not a doubt in my mind that it is real, but I'm playing devil's advocate here), that family is FUBAR. Not to say they already weren't, child abuse of any kind is inexcusable, but that added push of "Anon is having sex with his little sister" snaps them.
We're hardwired to think it's gross and icky, that it's a mortal sin that turns men into demons and makes God cry and whatnot. You gotta remember that, even though you're on a board where everyone's pretty much on the same page, we're in the societal minority here. We represent a tiny fraction of the world's population.
Yet it's joked about and thought about more often today . Tumblr has scores of people shipping incest and more people are coming forward with stories, both real and fictional, of incestuous relationships (myself included, but this isn't a post about that). The theory of it is accepted if only slightly, but the practice is forbidden and will probably remain that way until the apocalypse forces us to fuck our sisters and mothers to repopulate the world. Sad but true. Or who knows? Maybe rampant PC culture will suddenly deem incest a just cause and start fighting for us.
TL,DR; Good in theory, bad in practice.