What we basically did in migrating from the World Map to the Capital City is to take all the cities from the world map and group them together by type (you may recall that on the World Map there was more than one "Peaceful Valley", more than one "Standard Map", etc.). Within each group we picked one city to be saved (except for the "Luxury" and "No Storehouse" groups, in which no cities were saved) and the rest to be eliminated. The people who happened to be playing the saved cities are the people who are able to visit the Capital City immediately. The people who happened to be playing the eliminated cities are the ones who can't visit the Capital City without finishing their in-progress city (because their in-progress city would be eliminated and their in-progress game thus lost).
If you happen to be playing in an eliminated city, don't worry; the Capital City will be around for a long time to come.