Because it doesn't necessarily solve the problem of car journeys but costs a load. It might be fine for e.g. London but where I live, there is no public transport between my house and work (well, perhaps over about 3 hours compared to 30 minute drive) but even though my wife lives on a bus route, the times are less than hourly so she can't get to work for the right time, would get there an hour early or an hour and a half later!

If people still need to use their cars, it doesn't really work.