I do question the legitimacy of Neilsen ratings numbers, but, honestly there has been cancellations questioned, and when investigated, it turns out the ratings were correct, and the show really wasn't being watched enough to justify it being continued. And again, if you're not watching it live as it airs, but, instead watch it On Demand, torrent, streaming, after 3 days on DVR, etc, you can often avoid the commercials, and the folks paying the bills for the show want their commercials to be seen, that's what they are paying for. (Though On Demand is starting to show the commercials in the first week, with no Fast forward option, and DVRs also are starting to prevent you from skipping the commercials)
I use to question the Nielsen ratings as well, but realized there had been studies (pre-internet, God I'm old) that showed they were pretty accurate on what people were watching.
People like for the things their involved with to be popular (I know I do). But the fact is, science fiction has rarely drawn well in the ratings. When you factor in that it usually costs a lot more to make than a sitcom, drama or reality show it doesn't make economic sense for content providers to produce it.