One problem is that cars take up the space that other transit options need, or alternately, cars themselves are the reason other transit options are unattractive.

For example:

- many people are afraid to ride bicycles because they are (justifiably) afraid of being run over by cars

- separated bike lanes cannot be created because the space is already used for parking spaces for cars

- street level trolleys or buses can move people very efficiently, but when cars block their path, they are slowed to the same pace as the traffic jam

I could go on and on and on. Unless you take back the space used for cars, it is very difficult to make space for anything else.