How a literary work becomes considered great is part of a subdiscipline of literary criticism called “reception studies” because it is not so much a critical question about the poem itself but rather a question about how readers and critics have responded to the poem over time.
Two of the factors influencing Prufrock’s reception were its form and its portrait of modernity. In form, it was one of the earliest modernist poems, and along with Eliot’s Wasteland and Pound’s Cantos exemplified the fragmented presence of traditional meter, extremely frequent use of allusion to very specific details of many cultural traditions rather than to the earlier knowledge base of classics and Bible, the presence of multiple voices, and surface difficulty. In theme, it had a modern anti-hero and dealt with fragmentation of society. Combining both these innovations in a single poem made it a touchstone for the era, and thus “great.”