CalGal has it right.
Remembering back to the earliest days, it was how the progression of the individual boards developed. It was indeed originally solely a place for WDW. Pete lives in Central Florida and started the site from a passion for the place he enjoys most. From there, a few boards were added for other area attrations that WDW visitors might also be inclinded to go see while on vacation at WDW. Then, as more diversity of boards were added, a place for Disneyland came in.
Like CalGal said, there were very few posts on the Disneyland board in its infancy. I suppose laughing place was getting all the traffic.
While I agree that Disneyland certainly deserves higher standing, it seems that the general purpose of the boards is the same as it was originally - a place to discuss vacationing at WDW, and possibly other central Florida locations. So, I guess it will likely remain as is.