there are plenty of reasons why NY rap is not as good as it used to be but first, i disagree with your premise although you can argue that there are few venues for burgeoning rappers in the tri state area
but work ethic is a good reason....Ferg, Rocky, French, Troy Ass, Dave East, Flatbush Zombies, Bas, Nicki, Fab, all them are consistently dropping music and doing shows globally so i don't see the argument that Migos , Boosie etc are working harder than them. dudes in the South have certain advantages: 1) their style is popular/they can make whatever type music they want without their cities turning against them for "sounding like" another city 2)radio support/strip clubs 3) support from their legends/veterans 4)you can build a huge buzz just touring the South and Midwest...an ATL rapper can do shows in ATL, Birmingham, Florida, Texas, Virginia, Chicago, Detroit, Missouri, South Caroline, North Carolina...these states all have large populations and venues that smaller rappers can use to build a fanbase....an equivalent rapper in NY is going where? a couple small place in NYC, North NJ, CT, PA and thats it really as far as the Northeast
this is all my opinion tho....