Overview: Busking in the US can be ok; can be tough. So, where do you go busking in Europe? Where it's quite and people are handing out money.
Europe has a longer history than the U.S of being civilized. Europe has an overriding sentiment of kindness towards the human being. Busking, performing in the street for money, is of course more possible in Europe.
Both cities and town in Europe are designed to serve function of people walking around to shop for bread, cheese, and eggs and to walk to church. Even with the redesigning of most big European cities during the 1870's-90's the redesigned cities retain a space for the delicacy of the human spirit.
US cities and the meta suburban crawl which characterizes them (this design flaw finds its root in the salary offered to city planners disqualifies qualified architects from applying) does not offer much of a forum for Busking, the Busker, or being happy in general.
You can't go busking if the street is too loud and Europe, generally, is far more quiet than, say, Key West, Florida. People talk quieter. The cars don't honk horns; people don't scream.
Resolve: Busking is a tough way but it's possible to eke out a living on the streets of Ireland, Spain and France.