Because he synthesized the neo-orthodoxy of Karl Barth and the social gospel of Adolf von Harnack. From wikipedia:
... while his Christocentric approach appeals to conservative, confession-minded Protestants, his commitment to justice and ideas about "religionless Christianity" are emphasized by liberal Protestants ...
[In Harlem], he heard Adam Clayton Powell, Sr., preach the Gospel of Social Justice and became sensitive to not only social injustices experienced by minorities but also the ineptitude of the church to bring about integration. Bonhoeffer began to see things "from below" — from the perspective of those who suffer oppression. ...
In his prison letters, Bonhoeffer raised tantalizing questions about the role of Christianity and the church in a "world come of age", where human beings no longer need a metaphysical God as a stop-gap to human limitations; and mused about the emergence of a "religionless Christianity", where God would be unclouded from metaphysical constructs of the previous 1900 years.
