Why is that necessary?

Sort of... the state of the genre right now is a bit uncertain. There are currently a couple of city sim/city builder games on the market, but all of them have some drawbacks that make them unattractive to one segment or another of the city sim fan community. The reigning gold standard is still Sim City 4, which has an excellent and compelling core game, but which is now seven years old and starting to show its age graphically. The other current contender is Monte Cristo's Cities XL, but although it boasts modern graphics and some other features like freeform roads, it's been perceived by many Sim City fans as lacking in other areas (mass transit options and dynamic development, for example.)
 
In addition to these two, there are a number of more specialized games, most of them more limited in scope and intent: There are several titles focused on ancient world cities, for example, (Grand Ages: Rome, Children of the Nile) or specific geographic areas (the Tropico series). We believe that, as of now, there is no modern game on the level of Sim City 4, and that Sim City 4 has a limited lifespan for the future.