Back then, most people used CRT displays, and just like today, tended to have the brightness turned up too high. And it was fairly common to have a 30 Hz refresh rate. The refresh might even be interlaced!
A white background in this situation could lead to noticeable flicker, especially as you moved your eyes, e.g. look away from the monitor and look back at it.
The gray background made this flicker less noticeable.
A white background in this situation could lead to noticeable flicker, especially as you moved your eyes, e.g. look away from the monitor and look back at it.
The gray background made this flicker less noticeable.
https://en.wikipedia.org/wiki/Refresh_rate