Why did Pius XI exclude Eugenio Pacelli from preparing a planned encyclical against racism?

Pope Pius XI excluded Eugenio Pacelli (the future Pope Pius XII) from the preparation of a planned encyclical against racism because he was aware of Pacelli’s strong desire to prevent a break in diplomatic relations between the Vatican and Berlin.

While Pacelli had previously helped draft the anti-Nazi encyclical Mit brennender Sorge in 1937, his cautious and diplomatic nature contrasted with the more confrontational approach favored by Pius XI. Consequently, to ensure the development of an encyclical that would explicitly demonstrate the incompatibility of Catholicism and racism, the pope commissioned the American Jesuit John La Farge to prepare the document instead.1