Two main reasons SASless - the first is response time, If the climbers are stuck or hypothermic or slightly injured, perhaps just plain lost - their condition may worsen or they could step off a cliff.
Secondly, it is always hazardous to be on the mountain in those conditions and the mountain rescue team put themselves at risk - if that risk can be minimised or removed by using the helicopter then they are free to respond to any other calls. Even though they know the ground well it is still a bad place to be on a mountain at night in the dark with crap weather.