[Ekaros]

If it was about discouraging why not total ban of all non-emergency vehicles? Or maybe allow busses and trucks, but absolutely nothing else? That would solve problem much more effectively.

Or maybe first come first serve system. If too many vehicles are inside border new ones have to wait until one leaves?

[Freak_NL]

Probably because congestion charges are a proven system. You would have to ask the city of New York for their exact reasoning of course.