Speculate on why today Keats’s longer poems are typically not seen as significant as his shorter poems. Are the shorter odes and narratives “better” or are they just more accessible to today’s readers? What does looking at the reception of the longer poems tell us about this critical debate?
For Keats, death was always a part of his life, something that he dreaded, and something that made him write all the more desperately while he was alive. He lived a short life but was very productive in the years that he wrote. It is probably a coincidence that his shorter poems seem to have proportionately more weight than his longer poems - that is, it is probably a coincidence that Keats put more philosophical wondering in the shorter poems, as if he needed to pack them full of meaning given their brevity (like his life).
One could also argue that since the shorter poems tend to dwell on ideas/ideals and the longer poems tell stories ("Endymion" and "The Eve of St. Agnes"), there is more speculation on what the shorter poems mean. In other words, with the shorter, more contemplative poems, there is more room for interpretation ad argument about intended and potential meaning. For example, the argument over the final lines of "Ode on a Grecian Urn" ("Beauty is truth, truth beauty,"--that is all / Ye know on earth, and all ye need to know.") has never been settled.
"The Eve of St. Agnes" is considered a masterpiece but, being more of a Romantic narrative (and somewhat based on Romeo and Juliet), it lacks the originality and Romanticist contemplation of a poem like "Ode on Melancholy." In fact, in terms of originality, Keats himself was critical of another of his earlier long poems, "Endymion" calling it a "trial of invention." Still, he did want to practice the art of the Miltonic epic poem, so he began working on "Hyperion." But, once recognizing that he might simply be copying the Miltonic style, he abandoned the project, stating, "I will write independently." Keats was very aware of the danger of imitation and desperately wanted to be original. He would later attempt to finish "Hyperion" - then called The Fall of Hyperion.
Another reason the shorter poems are studied more than the longer (especially the epic poems) is that the shorter poems are more indicative of the Romantic period. And the odes (particularly the more famous odes of 1819) are characteristically more "Keatsian" meaning that they deal with Romantic notions that Keats was constantly playing with: mortality/immortality, beauty, transience, and the role of the poet. One way to think about this is that with the longer poems, Keats was circling around poetic and philosophical ideas but this was in the process of telling a story. With the shorter poems, he is addressing the ideas head on; this forces the reader to dwell more on the themes and the poetry itself, less on a story.