This is due to the much greater incidence of icing occurring at low thrust settings.
Really?
It's because, as you are descending, the chance of forgetting to turn on the ice when the temperature warms beyond the -40ºC limit is to great to worry about the tiny amount of extra fuel it takes to turn it on as you enter the cloud.
Entering the cloud is a strong trigger - the temperature warming past -40ºC is a very weak one.