Why?

The Benedictines of Ettal were opposed to Nazism. One of them, Father Johannes, maintained a close contact with the conspirators. So Bonhoeffer would be in surroundings in which he would not have to become involved in conversations with people he did not know. Though he was good at finding his way quickly in changed situations, his new duties were unfamiliar and he needed some quiet in order to prepare for them. As for his theological work, there was no better place than the monastery, with its sizable library. At night he would not be disturbed by air raid sirens, and no would ask out of curiosity what a healthy man was doing out of uniform in Munich in the middle of a war. There were eventually some questions asked, even in the little town of Ettal, but the monks knew how to handle them.

(Ferdinand SchlingensiepenDietrich Bonhoeffer 1906-1945: Martyr, Thinker, Man of Resistance, 252).