Because when the eyes of the Prophet (ﷺ) sleep, his heart remains awake. In the same way, there is a difference between the Prophet (ﷺ) and the Ummah regarding the obligation of ablution after sleep. For the Ummah, ablution becomes obligatory after sleep, but not for the Prophet (ﷺ).
It is narrated from Sayyiduna Abu Hurairah (may Allah be pleased with him) that the Prophet (peace and blessings of Allah be upon him) said: "My eyes sleep but my heart does not sleep."
Hadith Referenceصحيح ابن خزيمه / جماع أبواب الأفعال اللواتي لا توجب الوضوء / 48
Hadith Gradingالألبانی:اسناده صحيح
Hadith Takhrij«أحمد: 251/1 ، 438 ، وابن حبان فى صحيحة: 6386 ، و ابن الجارود فى المنتقى: 16/1 ، رقم: 12 ، من طريق يحيىٰ بن سعيد عن ابن عجلان ، الجامع الصغير: 2367»