NoAgenda,
Because it was obvious to Nick that Gatsby wanted to pay him for the service of arranging meetings with Daisy. The idea was distasteful to Nick, although he admits to himself that if Gatsby had made the offer under different circumstances, perhaps at a later time, or if the offer had been connected to some business deal and not so obviously connected with Daisy, he might have accepted it. Nick has some character flaws but is honestly revolted by the thought of being a pimp for Gatsby.