Ah, but imagine you were bidding Chicago in a year where the selection was in San Jose, while Anaheim also had a bid. Don't you think Anaheim would have a distinct advantage over you solely based on people living in the Bay Area saying, "Hey, I can drive down there in less than eight hours, whereas Chicago probably means having to fly!"

What we have is a compromise, but I think it's a good one and it does have a self-limiting action in that it's impossible to have more than two Worldcons in a row nearby each other. (Theoretically, the same site/committee could run two in a row, but aside from being insane, it's unlikely that the electorate would let them do it.) It forces geographic diversity, but doesn't guarantee that any particular region of the USA (or anywhere else in the world) will host the convention every N years.

And I'm not convinced that the three zones we had were ideal geographic rotation, anyway. How tighly tied is Chicago to Texas, or Atlanta to Boston?