How about because it didn't respect it's core audience demographic? It's an action/sci-fi series the main demographic is young adult. While gender isn't as important as it used to be it is still more male than female. Go write some fanfic, maybe it will be better than the source material.

Farscape absolutely the best use of Muppets for an adult marketed show! Brian Henson did amazing things with this.

Firefly also a Joss Whedon creation (lots of Bufffy actors on it) was brilliant. If it had aired in the proper order I think it would have been very well received.

Nearly forgot Supernatural! Absolutely great show!

Fantasy does not mean that science fiction is not also fantasy I would argue that these all cross back and forth.