Of course, most post-avantistes today consider the pentameter, along with everything else, broken a long time ago, and why not.

Pragmatics is a branch of contemporary linguistics; it emphasizes language as communicative utterance. One of the things it looks at are the pre-suppositions of a speaker regarding his or her audience, which influence all the choices of vocabulary & style. If you looked at the attitudes of various modern & contemporary poets toward style through the lens of pragmatics (as R.A. York did in The Poem as Utterance, back in the 80s), you could interpret the style of their "utterances" as motivated by social attitudes.

Why would a poet like Crane not break the pentameter, to take a salient instance? Perhaps because the social attitudes motivating wide ranges of modernist experiment - such as "epater le bourgeoisie" (then) or "disrupt oppressive social systems based on standardized syntax & meaning" (not too long ago) - held little value for him. The simplistic binary polemics which offer a bourgeois-sellout "them" vs. an aesthetic-political vanguard "us" are based on attitudes which severely warp the reception/absorption of the main stream of poetry in English; this has its consequences for practice.