Because it is not about "removing" the car, but making it the most difficult thing to use vs any other type of transport. For example, you can make the most direct route to a place via bike or walking, but if you use a car you have to use the ring road to get from one side of the city to the other. You enable cars to go into a city, but not through a city.

The other issue is that people equate the idea that people can walk and cycle together. This is really uncomfortable for walkers. You need to segregate space for each type of transport.

Cars are local optima. In a 100% car city you need a lot of car infrastructure and there is pressure to keep building more. In a 25% car city the pressure switches to maximizing other modes of transit, but as long as 98% car city is worse it's really hard for cities to make that jump.

PS: It's really a lot like social networks, dropping G+ vs Facebook is really more a question of which your friends spend time on not which one has better software.