Because danbooru posters love to give good scores to crappy hentai and mysteriously vote down perfectly normal pictures. They also seem to be into beastiality, which is annoying. I like a lot of things about Danbooru (the tags above all), but I find myself really missing the separation of /d/ type content on 4ch.
Blacklisting tags (particularly crap like like semenonfigure, beastiality, futanari) is nice, but it doesn't help that it also ignores normal pictures that are just inexplicably voted down. I guess I need to set my threshhold even lower...