Though Spenser was well read in classical literature, scholars have noted that his poetry does not rehash tradition, but rather is distinctly his. This individuality may have resulted, to some extent, from a lack of comprehension of the classics.
This is Wikipedia putting it crassly, but it's just the kind of attitude that put me off Chaucerian criticism when I was an undergraduate. The notes to my edition of Chaucer were nothing but a list of references to what he'd borrowed from Virgil, Machaut, Ovid, Macrobius, etc. - and if there was a line or an idea without a source, something that might even be deemed original, you could just feel the frustration steaming from the the page. The thought that poets might think of something for themselves was clearly disturbing, threatening even, for then what would become of the poor scholar? Ergo, if Spenser is not sufficiently slavish in his imitation, the readiest explanation is incompetence.
Romantic critics didn't seem to have the same obsession. On the contrary, they valued novelty and genius, even where, as in the case of The Lyrical Ballads, it was presented by the poets themselves as something of a return to an existing folk tradition. (And of course such declarations of a return to simpler roots constitute a tradition in themselves: cf. Thomas Sprat extolling the Royal Society in 1667 for its "constant Resolution, to reject the amplifications, digressions, and swellings of style: to return back to the primitive purity, and shortness".)
It's hard to avoid the suspicion that both sets of critics were influenced by the canons of the times they studied. Chaucer and Spenser, along with their contemporaries, set great store by authority and precedent; the Romantic poets emphasized genius and creativity (I simplify just a bit!). Their critics were perhaps drawn to these eras by a compatibility in their own temperaments and tastes, which were reinforced by what they found there; or else their nature was "subdued / to what it works in, like the dyer's hand". Either way, they needed to get out more.
Thomas Nashe was more of my mind, referring to Spenser in the splendidly-titled Have With You to Saffron Walden (a pamphlet written against Spenser's best friend, as it happens, so he had no cause to flatter) as "the Summ' tot' of whatsoeuer can be said of sharpe inuention and scholarship". A fine summation itself, and one that pays due regard to both these complementary, essential qualities.