New York was critically late in implementing any type of lockdown/social distancing effort. That, combined with their population density, average age (top 10 oldest in the US), weather, and mass transit use, it was just the perfect recipe for this thing to go nuclear there.
Louisiana was "densely populated" for Mardi Gras, so that could explain why spread is taking off there. I also looked up how the state deals with the flu, and they seem to get hit particularly hard by it vs. other Southern states. Don't know if it's because of their healthcare infrastructure, lifestyle (smoking, beetus, obesity, etc), and such, but flu death rates are correlating pretty closely with how hard a region gets hit. Ex. Two weeks ago I made that flu/covid thread, and found New York's flu infection rate to be 7.5x higher than both Cali and Texas. NY's death rate is 10x than Cali and 35x higher than Texas.
As far as different strains, this scientist sheds light.