There are more factors to consider as far as user experience than just what is "easier" on the eyes.
There are a couple advantages to black on white. One is printing. If I find a really good snippet, I might want to print it out and pin in on the wall next to my workstation for handy reference. I might also print out a longer article to read on the bus/airplane/in bed/wherever so I can take notes and highlight on it as I read. Those are just a couple of many use cases for why a user would want to print. But in general, black on white will use significantly less ink or toner. If you do white on black, certain browsers will print the text in pale gray on white (because it's default is to not print the background color). Not legible.
So you go ahead and say, great, I want to make it print nicer, I'll make a print stylesheet with inverted colors. It will only be white on black on the screen, when you print the colors will be reversed to print well.
Second use case. I actually do this a lot. I'm trying to gather a bunch of information on a particular topic, so I copy and paste segments that are relevant into a word document. I then later, condense it down into notes or whatever. When you use "reverse" colors, I get annoyed. Instead of code that's pretty-printed in IDE-like colors that will be legible in a word document, I have colors that are hard to read on a white background and print poorly, and I'm going to have to manually change all the colors.
Additionally, choice of colors may to some people be an indicator of professionalism or conformance to expectation. Lots of news sites, professional sites, even stack exchange, for better or worse, use white on black, not the reverse.
I realize color preference here is a strong preference for a lot of people. Some like it one way, others like it the other. Those who prefer their computer screen mostly dark with light color text have, in my experience, a very strong preference. In the other direction degree of conviction about their preference may vary. I have no statistics on this as to which is more popular, but it's certainly quite subjective.
Since you appear to really prefer light on dark, I'll point out that it is not difficult to accommodate both preferences. Swapping style-sheets at runtime, or having a user cookie to set style sheet preference is easy to do. Making your images look great on both light and dark backgrounds is slightly more work, but is not impossible either, so long as the consideration is made as the images are created so you don't end up with "glowing shadows" or the like.
And as others have already pointed out, inverting your screen colors is always an option, should you go with the color scheme you don't prefer.