Manhattan is 8th in the world for population density and wealthy enough to widespread test for the virus. Port-au-Prince, which has similar population density but a limited ability to test for the virus.
en.wikipedia.org
I think that cross between wealth and density is the reason why Manhattan exploded.
The mystery is Philippines. They have four cities more dense than Manhattan, not a great deal of wealth, but get supported by USA. Logic dictates they should have four hotspots exploding similar to NYC but they don't...yet...and the reason might be as simple as NYC being the trade hub of the world. NYC probably has all of the main genotypes of the virus bouncing around in it because it accepts goods and people from everywhere.
...it might be coming for Philippines. Headline as of 2 hours ago:
Philippines has temporarily barred doctors, nurses from leaving for work overseas
gulfnews.com
...also, NYC was really late to lockdown. The governor made the mayor do it.