Because informing the people of the mosque with the voice of someone consistent is more effective than with the voice of someone they are not accustomed to, so it is better. If a market vendor calls the adhan for the neighborhood mosque at night prayers, and someone else during the day prayers, it is permissible; because the market vendor is burdened by returning to the neighborhood at the time of each prayer due to his need to earn a living. See: Al-Bada'i 1: 149-152.