The problem with sff is poor casting, weak characters no one cares about, bad writing, recycling stories we've all seen before and in recent years thanks to LOST a tendency to create an unnecessarily complicated convoluted mythology that adds up to nothing in the end--The Event, V, Flash Forward, post S1 Heroes etc. And in some cases like BSG and Caprica the shows had a habit of creating unlikeable characters, too much angst and being overly pretentious which turns off viewers.
I think people will be more than willing to give a new show a little bit of time to find itself just so long as they like the characters and the premise--they'll wait for the writers to find their footing. But a lot of these shows give us characters we don't care for so then all that is left is plot and then in those cases you have to leave the gate running strong and most of these shows just don't--hence the erosion.