1. They don't know the lay of the land too well and don't have connections to establish bread-and-butter street rackets.

2. Street gangs and other ethnic OC groups have beat them to the punch. Why would those group work with out-of-towners in shy-locking and traditional mob rackets when they're already making a fortune on drugs?

3. No one to recruit once they get there, gangs thrive on personal ties, people they can trust to not rat or be too greedy. Who's going to be loyal to a bunch of guys sail in to town from New York. See point 1 also. They'd have to import soldiers, weakening already weakened families.

4. They're struggling to maintain what they already have. Who's going to stretch their resources on a big gamble in an area where cosa nostra has already been wiped-out.

5. Not too many major cities have a cohesive Italian emigrant population (or Sicilian, if the family is old school) like New York does. The ones that do still have an active cosa nostra group.