Why not just wait for someone to make one?
Surely game companies will develop a new game if there's a market for it.
Well, first of all, we're the impatient type. Secondly, EA (owner of the Sim City franchise) hasn't shown any sign that they intend to continue the series, at least not in the direction fans of Sim City 4 would like to see. Nor (to our knowledge) are there any other games of this type currently in development. Third, the genre has not had a great track record for commercial developers recently: EA's sequel/spinoff Sim City Societies fared very poorly, and the original publisher of Cities XL went bankrupt after its release. There is a strong possibility that investors and publishers will be wary of trying to trying to capture the city sim market for some time. Last, and perhaps most important, we believe an open source game built by the community will ultimately be a better game than a proprietary, closed platform.