Right, Andrew. It was a combination of things, though the main thing was Evans refusal to follow Stryker's "shooting scripts." The other main thing was that Evans's output, in numbers, though certainly not in quality, was below the output of most of the other photographers.
I'm with Evans, who's always been my favorite photographer, but I also have to go easy on Stryker when I consider who he was and what his position was in the New Deal organization. Stryker's boss, Rex Tugwell, billed as an economist, really was nothing more than a socialist politician, and Stryker was under constant pressure to produce propaganda that would boost the image of the FDR administration and minimize the PR results of its disastrous economic policies. If you do some serious reading about the FSA you realize that even though Stryker was way out of his depth and far beyond his area of competence, he did an absolutely amazing job building the FSA photographic file. All things considered I have to give the man a thumbs-up.
For anyone who's actually interested I'd recommend A Vision Shared: A Classic Portrait of America and its People, 1935-1943 by Arthur Rothstein, who was one of the longest-term FSA employees, and, later in his term of employment, a very fine photographer. There are other excellent books on the subject, but to me, this one is top of the stack.