So why do they keep doing this? Wikipedia's not a physical book. If I want to read an article about, say, limestone, I don't have to flip through hundreds of pages about webcomics and rock bands and Australian cricketers and fancruft. I just type "limestone" in the search box, and it takes me right there. How does the existence of 9,718 articles about anime and Pixar films, even if most of it is fancruft, hurt this process? It doesn't. Perhaps it might slow the search down a tad (but hey, if they're serious about hosting an encyclopedia, one hopes they'd have a nice big beefy search engine). Perhaps I might get a disambiguation page with a whole bunch of silly things on it, like "Limestone may refer to: A mineral. A webcomic. A rock band. The Limestoneers, another rock band. Harry Limestone, an Australian cricket player. etc." But still, hey, the first link in that list is the one I want, as usual, no problem! One extra click to get there is nothin'.
One possibility is that they're running short on server space. Granted, not everybody can set up huge server farms like LiveJournal or YouTube. But seriously, if they're that hard up, I wish they'd just come out and say so.
Or perhaps they want to be taken seriously by traditional media. If so: c'mon, guys, you're "the online encyclopedia anyone can edit". Quit acting like you're the frigging Encyclopedia Britannica.
Or maybe they don't want redundancy. I hear there's a wiki for webcomics called Comixpedia, and maybe the Wikipedia editors think articles about webcomics belong there instead. But once again, if that's the case, I wish they'd come out and say so. "It is recommended that this article be moved to Comixpedia" would be so much more useful and polite than "This article has been BALEETED."
Or maybe some editor is just deleting stuff he doesn't care about. In which case, what a jerk.
Actually, I can think of one Wikipedia feature that would be hurt by too many non-notable articles and fancruft: the Random Article link. But who even uses that, unless maybe they're bored?