Because the problem wasn't "ice," the problem was "cold," especially when you're dealing with finicky moving parts regarding pressure switches and tilt switches. There are tons of components of the era that were subject to thermal variance: oil-and-paper capacitors, wire coils in transformers and inductors, etc etc. We take solid-state reliability for granted these days, but there was a time before.

For once I applaud the British and their shitting engineering!