Because cars in Manhattan are a blight on livability and safety. There are only a handful of reasons to legitimately have a car in Manhattan; off the top of my head: moving large items, transporting those with mobility issues, getting from specific subway dead zones to other parts, and leaving manhattan altogether. There is an enormous (albeit overloaded and dilapidated) public transit infrastructure allowing people to both move throughout the city and come into the city from basically anywhere in the tri state area. And even if direct transit options into the city aren’t spectacular, you can drive to any of the thousands of park-and-ride locations on the cheap and take public transit from there.
There are legitimate life circumstances that necessitate driving around manhattan, and I would be more than happy to exempt those from congestion pricing. And I have no problem with Taxis and buses and delivery vans operating, as they have high utilization. But private cars cause so many problems in manhattan, including danger to pedestrians (which make up the bulk of people in the city at any given time), noise pollution, air pollution, and inefficiency for legitimate ground transport/logistics, that I have no problem disincentivizing the use of them in Manhattan with large tolls. There’s a certain hubris required to be the only occupant in a vehicle in manhattan when the public transit infrastructure is so expansive and affordable.
And don’t even get me started on all the wasted space dedicated to parking that could be used for public space, business, bicycle lanes, etc...