Seven million people commute into Manhattan each day. Almost two million live on the island. Few of the residents commute out of the city to work. This means that on the average work day there are at least 8mm people on an island that is 13 miles long and 8 miles wide at its widest point. A groundburst anywhere in midtown would almost certainly kill a million people.
Correction: The widest point of Manhattan is 2 miles.