Style. Ferraris are red (usually). Formal ties are black (mostly). Race cars other than English ones are never green (that is changing). Computer cases? Too early in their history to have a preference. Might have had something to do with sales figures. A generation ago, when good laptops came from Japan, black seemed stylish for businessmen, not clashing with suits, briefcases, Cadillacs. That "high style" seems to have transferred to desktop computer boxes, monitors, keyboards. But Apple has a style of its own, still.
I have several colors of desktop equipment (white, putty or ivory, gray, silver, black), but my laptops since 1995 have been black, black, black.