This comes from David Perkins's A History of Modern Poetry: From the 1890s to the High Modernist Mode (1976; pgs 128-130; it is the first of a two volume history), closing off his discussion of Edward Arlington Robinson. The ideas he raises are both historical and theoretical in nature.
These long narratives go on mostly for between eighty and a hundred pages and make up abut two-thirds of Robinson's total output. They appear to have been admired mainly on principle and to have been more praised than read. Neither can one challenge this consensus of inattention. It is impossible to read them with more than languid interest. To ask why this is so, however, is to raise fundamental. questions about the limitations and the evolution of poetry in the modern world. Robinson, to be sure, was not a storyteller of Chaucerian genius. But since the Romantic period no poet has scored a major success in narrative poems of this length, though a good many have tried. One thinks of Browning's The Ring and the Book, Arnold's Sohrab and Rustum, the tales of Morris, and Yeats's Wanderings of Oisin. The short short story of a few hundred lines, such as Frost's "The Witch of Coos," or a series of linked lyrics making up a story, such as Meredith's Modern Love, seem to exhaust the possibilities open to poetry on this line. Why? Answers can only be speculative, and while we are speculating, we should keep in mind that the question of the long narrative poem overlaps the question of the long poem generally. Here again there have been few successes that cannot be reduced to the general form of a long poem made up out of linked short ones: for example, In Memoriam, The House of Life, The Waste Land, Four Quartets, The Bridge, Paterson. The few exceptions that spring to mind – the longer epistles of Auden or Ginsberg's Kaddish – are especially revealing: they succeed precisely because they avoid the more condensed uses of language that have been the norm of poetry for the last fifty years.