I read on Wiki that the straight razor handle is also called:
Schale - German, meaning shell (which is what the word scale is derived from).
Manche - French, meaning sleve
Cacha - Spanish, meaning handle
I bet people called the handle schales in the old days back in Germany and England and over time and with western movement it just became spelled scales.
So I'm going to go with the explanation that: They're called scales from a German term meaning shell. You could think of them as the razor case. Sounds good.