If you want a reason for why they are the FBI number 1 priority. They have some of the best forgers and printers in the world to be able to give some one a completely new identity, which is a threat to the NSA. Another reason, some of the groups have their own weapons factories, military grade weaponry that they are able to smuggle into the United States.

There is a large presence of Russian crime groups in Miami, but most of these are small groups who relocated to get out of the influence sphere of the larger groups, with the exception of both the Brother's Circle and the Grekov group from St. Petersburg, Russia. Also keep in mind many news reports get the Russian and Chechen mafia mixed up. There is a small presence of the Chechens in Miami who are a big mover in drugs for the other Chechens in latin american countries. Then you Have the Georgians, mainly from the Kutaisi clan or other groups that have become a force in the past two decades who operate in Florida. These three groups are different but are always thrown in together.

I still believe that New York City is still the center for the Russian Mob.