Missed your post, PG-but here is as much as I have time for in reply--

Brecht wasn't actually all that welcome in Switzerland, and he moved to Austria, where he was granted citizenship, but there was a great backlash when it was discovered. When he went back to Berlin, he was basically back home, and it was a home that had been devasted by the thing that he had fought against up until he had been forced to leave.

To personalize it a bit, Berlin after the war probably seemed a lot like New Orleans after Katrina. As an artist, as an advocate of social justice, as a human being, he was compelled to accept whatever he found and to use it as a starting place.

He was a poet and a playwright--he used Marx as a way to look at the dynamic of society, and and the economic system that sustained it, but he was a social moralist, not a Communist--he wanted people to see the truth about the world that they lived in, and, by helping them to understand it, he hoped that they could unite and build a better world.

In that sense, ultimately, he had to be back in Berlin.