I suspect the decline is largely related to the Internet and the increased ability of people to work from home. Work from home = no commuting = less miles driven.
EDIT: In fact, most do. = 46.6%. Happy?
23% in manhattan, ~46% overall. Definitely not a majority, especially if just talking manhattan.
New York City is the only city in the country in which more than 50% of residents don't own a car, and that's looking at all five boroughts: https://en.wikipedia.org/wiki/List_of_U.S._cities_with_most_...
(If you were looking at just Manhattan, the number would be around 80% - estimates vary.)
The next two are also in the tri-state area (Newark and Jersey City).
It's not until you get to DC (#4) in which you come to a different metropolitan area, and they only have 37% of residents without a car.