SciFi is niche to begin with. I can think of very few SciFi shows that ever achieved mainstream appeal (call it 3+ seasons).

Of course, when you think about it, what's the percentage of normal shows that actually get 3+ seasons?

The difference may not be the percentage but the rabidity of the fanbase.

Even the hardcore Whedon fans didn't like Dollhouse very much. Just a handful of episodes, really.

I haven't watched Agents of SHIELD at all yet, because the early episodes have had an even worse reception on whedonesque.com, though that seems to be improving lately.