Because it involves carrying water that might not be used - at 10lbs weight per imperial gallon.
I've often thought that certain heavy lifting helis could use the water/alcohol windscreen washer fluid as required to:
A) Boost engine power for certain flight regimes / tasks.
B) Carry out a self compressor wash, to save the balls-ache of having the engineers come out at the end of the day to do it for us.
Or, C) If all else fails, wash the windscreens.