Shelley sure did love him some iambic pentameter (check out "Form and Meter" for specifics on the metrical form). And why not? It hearkens back to old Latin and Old French, and big-time greats from Chaucer to Shakespeare were fans. So naturally the Romantic poets, being way into Shakespeare and all things classical, utilized iambic pentameter pretty frequently. Check out "Ozymandias" for another example of his preferred style.