As well as being at full capacity, they are worried that the present bridge may collapse because it was built on the cheap and uses the same style as a bridge in Minneapolis which collapsed in 2007.
The location of this bridge was chosen because it was the closest they could get to New York City without the bridge coming under the Port Authority who would have demanded the tolls. Compare with the Thames where the City of London's control of the river extends a long way up and downstream from the city.