Good question.
Archbishop Sean is a member of a religious community - the OFM Capuchin (Order Franciscan Minors - Capuchin). O'Malley is a Capuchin Franciscan monk who lives under a vow of poverty and often wears the brown robe and sandals of his order. (You will notice the brown robe protruding in the above picture.)
It is the diocesan priest, formed by the diocese rather than a religious order, that wears the Roman collar and black suit. Archbishop Sean is only the 2nd bishop to be assigned by the pope to shepherd a diocese.
Okay ... there will be a quizz on this next week :-)