As much as, probably more than, aesthetic preferences, I would posit that there are many environmental differences which are difficult to completely control for. Screen type and configuration, ambient light level, light source positioning, how often the user look away from the screen (completely away from work, or just onto other media to read from a page for instance), etc.

Having said that I think my preferences are mainly, if not entirely, aesthetic. Dark-on-light terminals feel weird as do light-on-dark documents/spreadsheets - that can't be anything other than just what I'm used to.