Why wouldn't a agent want dual agency in real estate?

He/she is working as an Exclusive Buyer Agent, which forbids him/her to have any listings and cannot therefore act as an agent for a seller. Otherwise, he/she can be uncomfortable with the legal responsibility to act as an objective and neutral go-between for both the seller and buyer. This puts some limit on his/her relationship with one of the two parties. He/she is expressly forbidden from taking sides and favouring one of them against the other.