Despite commercial efforts (!), some people don't go mad about Halloween in the UK either - not like the USA anyway. It is steadily growing in popularity however.
I see your point about the Terrorific nights selling out but honestly, could they not even hold one or two
MNSSHP nights

: They didn't need to run as many and then perhaps they would find them full...?
I honestly wonder what Walt would have thought about the Terrorific nights - having seen the videos, I really don't see how it fits in with the Disney ethos... But I suppose holding them in the Studios fits better than the MK....