many of the suggestions are part of the right set of ideas, there was more than one reason, including mimicking paper. at that time many of these things were new and hadn't been done before, so they were done to show that it could be done or to see how it looked and it was fresh. (like the iphone's new liquid glass, is that really the best idea for a UI (feels it was only yesterday we got rid of the <blink> tag) or is it just new?)
so to add to the mix, that with a monochrome display, relatively low rez lighted pixels making letters (on a black background) still has the letters looking pixelated. the same lighted pixels making a white background leaves the black letters looking smoother.
outside of "labs", i remember the VT-100 being the first "white paper" display that was generally in use, but that could have been a regional thing in the shadow of the Maynard mill.
More or less, from what I've read. Once there wasn't a technical issue, a white (or near white) background became seen as more user-friendly than a black background.
