It's not difficult to understand the reasoning behind this. You've got to act as a matchmaker between two different cultures. You've got to have an understanding of the two cultures, including language, customs, and so forth. In addition, you have to be familiar with the social backgrounds and human interactions.
柯傅的理由不難理解：身為兩種不同文化「觸媒」的經紀人，首先要對兩種文化、包括語言、禮俗等，都有相當程度的瞭解；而且在兩個社會中，都要有足夠的「背景」、「人際關係」。