fashion, pure and simple. Chrome used to be the norm -- leica and Exakta all made chrome lenses.
But in the 50s and 60s photographers wanted black because they figured it made them less noticeable--suddenly black cameras and lenses were the preferred mode, Chrome less so.

Funny thing, for modern lenses with all their elements, black gets hot and expands too easily, which is why Canon makes their long lenses white.