Because of the historical accident of paper being white and ink being dark. The human eye is somewhat better at reading light-on-dark, but most people aren't used to it.
But there's also the historical accident of early computers primarily using light-on-dark displays (I always found amber-on-black mode less harsh on the eyes than green-on-black, personally—but just don't get me started on white-on-blue, bah!), which we then moved away from in favor of dark on light.
(If I had to guess the reason for that historical accident, I'd think it had to do with "white" not being a particularly uniform color on a lot of older CRTs, but one with varying hue and intensity in different regions.)
I find light on dark hard to read, even in low light. If I'm reading on my phone at night and use light-on-dark, the text looks blurrier to me than if I change it to dark-on-light (where "light" is a shade of gray that isn't harsh in a dark room).
(I wonder if dark ink on white paper took off over white ink/paint/whatever on dark paper because most people found it more readable. Anyone know about the history of paper/inks/paints?)
>If I had to guess the reason for that historical accident, I'd think it had to do with "white" not being a particularly uniform color on a lot of older CRTs, but one with varying hue and intensity in different regions.
also, the earlier monitors flickered more than later ones, and the flicker was more troublesome when the default/background color was light.
I think it's been overly stated in this thread, but it would appear that the majority of the problems with dark-on-white are the brightness, contrast and luminosity of the designs.
W3C apparently knew this was a problem and had provided an algorithm to measure relative contrast/luminosity, which is good to play around and test with: http://www.w3.org/TR/WCAG20/#contrast-ratiodef
I can read a dark-on-light text at normal size (on a retina display) for an hour without feeling my eyes are hurting, but I literally cannot read light-on-dark text more than 10 minutes and have to bump the font size one or two points...
