Because there are two types of Fajr: one Fajr rises during the night, and the other rises with the coming of the day.

It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said: "There are two types of Fajr. One Fajr is such that (for the fasting person) eating becomes forbidden in it and performing the prayer becomes permissible. The other Fajr is such that performing the Fajr prayer is forbidden in it while eating is permissible." Imam Abu Bakr (may Allah have mercy on him) says that in this hadith there is evidence that it is not permissible to perform the obligatory prayer before its time has begun. Imam Abu Bakr (may Allah have mercy on him) says that the statement of the Prophet (peace and blessings be upon him): "One Fajr is such that eating becomes forbidden in it," the Prophet (peace and blessings be upon him) means the fasting person. And in it, performing the prayer becomes permissible. The Prophet (peace and blessings be upon him) means the Fajr prayer. And one Fajr is such in which performing the prayer is forbidden. The Prophet (peace and blessings be upon him) means the Fajr prayer. Because when the first Fajr appears, at that time it is not permissible to perform the Fajr prayer because the first Fajr appears during the night. The Prophet's (peace and blessings be upon him) intention is not that at that time voluntary prayers cannot be performed. The Prophet's (peace and blessings be upon him) statement "and in it eating is permissible," he means the person who intends to fast, so he can eat. Imam Abu Bakr (may Allah have mercy on him) says that in the world, no one has narrated this report as marfu‘ except Abu Ahmad Zubayri.