because Germany has good laws and Berlin is a big city, yet EXTREMELY cheap.
The visa doesn't have all that much to do with it really. Most of the people in the US don' need a US visa, and guess what, more of the Europeans are close to Berlin. Crazy I know.
I'd say about half of my friends in the US are not US citizens, and their most frequent topic of conversation is visas. If you want to just show up in the US and work illegally, that's fine. But if you want to do it legally, visas are a big deal.
The visa doesn't have all that much to do with it really. Most of the people in the US don' need a US visa, and guess what, more of the Europeans are close to Berlin. Crazy I know.