Take a look at top programs in those areas, such as NYC.
Go to their residency profiles. Youll see a lot of folks from top tier med schools, and then you'll see folks from middle and low-tier medical schools that are either based in NY or have students who, in their profile, will say their hometown is NYC.
This is probably due to a mix of factors, though:
1) Regional preference as stated
2) These people are more likely to know people and have connections due to their ties
3) These people most likely have more reason to apply to these locations and also rank them higher