The two epics written by Homer the Illiad and the Odyssey are considered to be what's known as Primary epics. The third book written by Virgil, The Aeneid, is considered to be a secondary epic. Why is this?

Basically this is the difference:

For Primary Epic

No single author (each is a product of the oral tradition) Written down after centuries of oral transmissionSecondary Epic A single, gifted poet such as Virgil or Milton composes a work that imitates a folk epic. The Æneid and Paradise Lost, for example, involved considerable research and have the style of earlier epics (particularly in setting, dignified speeches, and extended similes.