[Someone]

In medicine, the slide projector was very common, because they tended to want to display photographs.

I think that’s what drove the defaults of PowerPoint, with its white text on a dark blue background (see https://www.edwardtufte.com/bboard/q-and-a-fetch-msg?msg_id=... for arguments as to why that’s good for slide projections. I think it also minimizes contrast differences between typical “medical conference” photo and text slides. That’s important in a darkened room , which they had to use because projectors weren’t bright enough yet.