Epic Poetry; Romance; Parable
In the ancient world, which gave us the genre of epic poetry, epic and romance did not go together. In fact, a famous Latin epic, the Aeneid of Virgil, depicts the epic hero rejecting romance and love in order to fulfill his heroic duties; in other words: romance and heroism are like oil and water.
Fast-forward hundreds of years to the Renaissance, when Spenser is writing, and that old division is way over. Romance and epics are all the rage, starting in Italy (famously in Ariosto's Orlando Furioso on which The Faerie Queene is heavily based) and coming to England. If anything, what we see in Spenser is an ideal of epic heroism in which romance is completely central—you can't really be a good knight if you're not fighting for the love of a worthy lady.
So while writing an epic romance is pretty conventional at this point, making that epic romance into an allegorical parable is pretty new. Allegory had been around for a very long time, but had never been embedded into a tale of knights, dragons, and castles as extensively as this. Indeed, romance involving knights and distressed ladies was considered slightly immoral, lowbrow entertainment reading. Thus, Spenser is doing some pretty interesting by taking this supposedly "superficial" genre of literature and using it as vehicle to impart moral and religious truths.