What does multiplayer city building even look like? I mean why and how would you want to interact with other people?
I am not 100% sure, but being able to trade oil and water and get mass transit certainly wasn't enough for an MMO price tag. Emperor had multiplayer trading which wasn't too bad but still a bit of a disconnect. The Settlers series always had some sort of multiplayer in it, right from the first game release in the 90s. It's kind of a city builder.
Really though I was thinking along the line of some friendly competition, liking winning bids and companies. If you are Seattle and you get Microsoft or Boeing to come into your city, then you are going to get some commuters from surrounding cities to go there but not a lot of them want to live there. It would be interesting to see if you had 2-5 people playing with cities connecting to one another how they would evolve side by side. It would never be a quick skirmish but I think a lot of city builders are very patient to play the games in the first place.
I mean Cities XL seemed to introduce an idea that I sort of liked too, cities specializing in stuff instead of just generic industry and resources which, in multiplayer, could make things intersting.
Just saying MC complete failure of this game doesn't mean that city builders can't have multiplayer; it just means you can't slap a MMO price tag on a single-player with a very weak multiplayer component.