1. The migration policy of the US - "Give us your poor, your hungry..."
2. The prohibition in the thirties created a huge demand.
3. All the other opportunities and space in the US (eg: The empty desert space where now Las Vegas can be found as a result of banned gambling in other States).

Now that the Berlin wall is no longer there, we get the same in Europe; lots of gangsters from Russia, Ukraine, former Yugoslavia, etc are now partying in Amsterdam.