Because while I don't watch the show myself, I go on FS daily and read all the threads about it, I have a large number of friends who are big fans and they talk about it with me, show me things related to the fandom, etc, and I have friends who do episode reviews on their LJs and I read those sometimes. I've also watched an episode or two at said friends' behest.

I'm fairly exposed to SPN fandom and I know a reasonable amount about the show, its plot, and many of the characters, but honestly, that's enough for me. I have no desire to sink a large portion of my time into watching six seasons of a show I only care passively about due to friends liking it, that has a record of bombing with female characters in one way or another. Quality of lady characters is incredibly important to me, and if I know going in that a show is lacking, welp. Chances are that will keep me from going in particularly far.