My cinema buddy's theory was that Steve and Tony had to do New York because they could use the power of being who they are there - ie people might listen to Stark and Cap. Bot sure about the rest. I mean Thor in Asgard makes sense, Nebula and Rocket on other planets makes sense and less likely to stick out, Bruce at a time when Hulk wouldn't stick out I guess since being green is now permenant...I guess the others got what was left?