While black on yellow isn't necessarily the best combination, you have to take into account the medium of the times.

On the typical 15" CRT with its high dot pitch and relatively dark tube that garish yellow would be far less garish. The black, by virtue of the display, would also not be as black and more a very deep grey. The text contrast would also be somewhat comfortable as it would get an anti-aliased effect even though the OS at the time didn't have good font smoothing at the time.