Let's face it.
By network standards all Sci-fi shows have been sorta ratings challenged. On cable the ratings expectations are lower and offset by the fact that you have a smart, educated audience with a high income.
Not really. X-Files was semi-sci-fi and got great ratings. Even the original BSG got great ratings but it cost so much to make it needed supergreat (like Superbowl ratings nearly) to be profitable. The DVD sales say Firefly would have gotten great ratings if Fox had only shown two consecutive episodes in the same time slot. STNG got great ratings all the way through it's run.
The big problem is the big network guys just don't get sci-fi. Generally when they sign on for a sci-fi show they're looking for lots of explosions. Then on Fox of course they have that desperate need to kill all their good shows with stupid decisions leaving more room for lame crap like The OC. One of the most telling facts of just how dumb network guys are when it comes to TV is how much they complained during the original BSG about Starbuck acting so manly and flirting and smoking cigars and stuff.