Another Explanation (and Oversimplification)
According to The Right Stuff by Tom Wolfe (an excellent read, okay movie), the TK pilots were presented with a problem, didn't cope with it, and crashed and got killed - they had the wrong stuff. The EK pilots were presented with a problem, handled it, and got back on the ground with nobody hurt - they had the right stuff. According to Wolfe's understanding and description of the test pilot psyche (and that of the race car driver, mountain climber, etc.), he (or she) copes with the risk of fatal failure by "knowing" that they have the right stuff and will not crash, whereas someone who crashes and is killed did not.