Here's a research project: twentieth century Spanish poetry that is [n]ot avant-garde even when critics call it that. Poetry that is radically modernist is in the vast minority here. The wind-through-the-poplars style* is much more prevalent quantitatively. I'm not talking about rescuing it from oblivion or saying that it is unjustly neglected, but of answering the question of why it is so prevalent once you take away the more canonical and avant-garde work. Even in other generations of Spanish poetry that old-fashioned romantic nature poetry is all over the place.
*I just made that up.