Just a speculation, thought:
I know that knives with low hardness steel (in HRC) are hard to sharpen perfectly.
Might be similar with bearing balls. The finer the grade, the more hardness might be required to enable bearings to be machined to that specification.

Softer balls are more easily damaged themselves - then damaging cups and cones more easily.
Too hard balls will also be more likely to damage cups and cones.
So I guess it's best to go with some middle value.