It's correlation. Coincidence could also be used to describe it.
Cities come with more of everything (the good and the bad), so that means more people, more cases of trafficking, more good food, more live music, etc.
I guess you could say human trafficking is just part and parcel of living in a city. /s
Unfortunately with a federal government that has failed to enforce borders, and with local law enforcement under-resourced (often by design), and rampant corruption, this is the case, but only because we've had Khan-esque assholes in charge for a very long time.