Because the Prophet Muhammad (peace be upon him) informed about performing the Zuhr prayer later during intense heat, and he also stated that if it were not for considering the weakness of the weak and the illness of the sick, he (peace be upon him) would have delayed the Isha prayer until midnight.

Sayyiduna Abu Dharr (may Allah be pleased with him) narrates that when the Mu’adhdhin of the Messenger of Allah (peace and blessings be upon him) called the adhan for Dhuhr, the Prophet (peace and blessings be upon him) said:“Cool it, cool it,”or he said,“Wait, wait.”Then he said:“Indeed, the severity of heat is from the steam of Hell, so when the heat is intense, cool the prayer.”Sayyiduna Abu Dharr (may Allah be pleased with him) says: (Therefore, we would perform the Dhuhr prayer) when we saw the shade of the mounds.