Because the Saints have a halo on his head?

The halo, known as halo, is the bright circle around the head of Jesus, the Virgin Mary, the saints and the blessed. He began to appear in Christian iconography from the fourth century A.D. as a symbol of bliss and heavenly glory. But it was already widely present in depictions of Egyptian art, Greek and Roman, in particular as an attribute of the gods of light. As a symbol of dignity and power also

appears in depictions of emperors.

(Look at the interactive history of religions and how the saints are made)

Not all haloes are equal

Initially the halo, in Christian representations, was reserved only to the figure of Jesus and was, at times, crossed from the cross. The halo without the cross, however, indicates a canonized saint, while if it is made in lines in the shape of rays denotes one of the blessed are not canonized. Variant of the halo is the almond, which encompasses the whole person and is reserved to the Virgin and the figure of God and always appears in scenes that highlight the divine nature of Christ.