Black background I guess hearkens back to the days of green screens and DOS: you don't waste light drawing anything you don't need, every pixel is precious. It took some time to realize that, actually, pixels weren't special at all, and something easier to read, like newsprint (color on white), is better.
Black is also easier for additive mixing, even if you aren't doing proper alpha blending. OR type blending was very common, back when 16 colors were common (and they weren't made of pixels, but bit planes).
Nowadays, these things are usually solved, but a lot of old code and ideas (like do-as-we-say, not customize-as-you-need) persist, especially among special-purpose software like this.
Tim