and programmability. Most publically manipulable stuff only gets hijacked by a few people with a bit of time and a few scripts. PitBull/Bieber "Where to perform next?" online polls, that ML Twitter bot MS recalled (was it Tammy or something?) etc were all attacked by spam bot style flooding of votes and messages to poison the data, and it's a lot easier to just spam incoherent unpleasantries than it is to spam coherent substantial messages.
AFAIK Reddit's /r/space didn't have any obvious vectors of attack so after the 1,000th manual entry of the n-word the trolls just got bored.