While it was in development, all the way up until when the pilot got picked up, I never once saw it labelled a "limited series", so I really think this is a matter of The CW having picked up 90% of their roster (some rather unjustifiably) before this show would even air an episode, and they would also need to account for new pilots coming down the line, so I think the network really sealed this show's fate before even giving it a shot. It was going to end regardless of turn-out.
But, all things considered, this was a more close-ended finale than a lot of shows that get cut off at the knees.