__Well, originally black backgrounded web pages were posted by people who believed that the Internet should remain free to use, i.e. you only pay for the connection not the content. Now it seems that most people do it to save your peepers from eyestrain. [Staring at a 60hz display will slow down your blinking until your eyeballs dry out .]
__As for white, typographers say that it makes the text easier to read. Also, you'll notice that places with lots of stuff to print (e-commerce sites, news services) generally have white or light colored pages.
__So why just black or white? Becuase it's garaunteed to be black or white on all computers.

I goofed!
__ Black meant that the peoples posting the page were advocating free speech on the Internet. This was back when the (US) government was considering censorship of the Net becuase too many parents were afraid their boys and girls would find porn, how to make bombs, etc. I guess some migh sill be keeping their pages black for people in places like China where there is censorship.
__Again, I appologize for the mis-info. I don't know where my brain was this morning,