Archaic, which is just a fancy word for "old-fashioned," is the number one stylistic quality of Spenser's Faerie Queene. Love it, hate it, or find it perplexing, Spenser was clearly up to something when he went seriously out of his way—and out of his own linguistic comfort zone—to write in a style that he thought sounded like the ye olde middle ages. (Spenser's archaic style is why it's a great idea to have an edition of The Faerie Queene with lots of footnotes.)
While there's no one answer for why Spenser would write this way—it's not like the Medieval chivalric romances he was imitating wrote in any style other than what was most current for them—it's clearly connected to Spenser's interest in constructing a pre-history of Britain and in transporting his reader's to another, older and remote world.
To put it in perspective, Spenser was writing stuff like:
Euen the famous Britomart it was,/ Whom straunge aduenture did from Britaine fet,/ To seeke her louer (loue farre sought alas,)/ Whose image she had seene in Venus looking glas. (III.i.8)
At roughly the same time Shakespeare was writing stuff like:
"Thus have you heard me sever'd from my bliss/ That by misfortune was my life prolong'd/ To tell sad stories of my own mishaps." (1.18.118)
No question about it: Spenser was writing in a ye olde-timey way by Shakespeare's standards.
Unsatisfied with just constructing the alternate, imaginary world of Faerie Land, Spenser wanted the unfamiliarity of the poem to be reflected in the very language itself, a constant reminder to the reader that no, you're not in Kansas anymore.