I actually got the good fortune to interview Bryan Fuller about it. Essentially, the network saw that there was a decline in quality from Season 1, one that wasn't going to get better with a third season, and decided not to pick up the show again. Plain and simple.
I'm sad there won't be any true closure for this show, but, even though there was, indeed, a lot of good in season 2, seeing what the show was becoming without Fuller's ideas, and with the wrong people making the decisions (namely, MGM), I'm glad they pulled out before any more damage was done. We're left, at least, with a thematically fulfilling ending, and I'll take that over watching the show get worse and worse in subsequent seasons.