Dr Who was always a Beeb production, but that didn't stop it deteriorating with age and neglect, twice now. SF has always been popular with Real People, and therefore a bit declassé, so it doesn't have sufficient snob value with Them Up Top. They love the money it brings in, but, this side of the pond at least, they can't bring themeselves to admit they like it for itself to each other.

Meanwhile, books by Agatha Christie continue to be televised, despite (as Terry Pratchett pointed out) the stories being set in just as unreal a setting as SF: it's just a fantasy about the past. Now if we could launch the idea that SF is reminiscing about the future .... :-)