He is called Vishnu because his energy pervades the whole world from the root Vis to 'enter' or 'pervade:' and all the celestials, the Manus, the kings of the gods are but the impersonations of the power of Vishnu.

In plain words: He is called Vishnu because his power fills and enters the whole world; the name comes from a root meaning "to enter." All the gods, the Manus (first rulers of each age), and the kings of gods are just forms of Vishnu's power.