Because people of all capabilities need to get to the businesses in the cities. If you're proposing prohibiting private transportation, you first need adequate public transportation, which is expensive and often lacking.
The alternative, where _nobody_ has easy access to the city's businesses, tends to result in the businesses moving out of the city to somewhere else, taking their tax payments with them, often places that are _even harder_ to get to for people who don't have cars... because businesses need customers. If you won't support them (by providing affordable and convenient means for the customers to reach the businesses), they won't wait to die, they will solve the problem themselves.
Only cities which have well-developed public transport networks, and have very popular city centres (to the point of congestion) are even thinking of banning cars from them. The rest of the cities need all the help they can get to fend off financial ruin from e-commerce.