Because the call to prayer is a revered remembrance, and performing it in a state of purity is closer to reverence. Abu Huraira, may Allah be pleased with him, said that the Prophet (peace be upon him) stated: ((Only a person in a state of ablution should call the prayer)), and in another narration: ((No one should call to prayer except in a state of ablution)), in Sunan al-Tirmidhi 1: 390, and he said: This is more authentic than the first hadith. And from Abdul Jabbar bin Wa'il from his father who said: ((A right and established tradition: that a man should not call to prayer unless he is in a state of purity, and should not call unless he is standing)), in Sunan al-Bayhaqi al-Kubra 1: 392, and Musannaf Abdul Razzaq 1: 465, and in al-Talkhis 1: 205: Its chain is good except that it has a discontinuity. See: Fath Bab al-Inayah p1: 208.