This is a question I’ve been pondering for awhile. It’s not as though there is a formal rule stated in those terms, or even an informal understanding. Rather, the almost exclusive of discursive prose is a consequence of something else. What?
In a post from October I observe that criticism “is couched in terms more or less commensurate with life those we (that is, literary critics) use to understand life itself (how we live)”. And that leads to so-called Theory, where the critic takes theoretical discourses originally oriented toward society, or the mind, or even (philosophically) toward the world at large and applies them to literary texts and, in cultural studies, toward other kinds of “texts” as well.
Now we’re getting somewhere. Let’s take it one step further. Is the literary critic more like a priest or an anthropologist? The anthropologist has no direct stake in the materials they examine, for the anthropologist is an outsider to the society. The priest, in contrast, is an insider, even the ultimate insider. The priest interprets the sacred texts so that their people might know how properly to live and how to understand the world about them.
It seems clear the core of literary criticism is more priestly than ethnographic. That’s why the question of determinate meaning was so important in the debates of the 1960s and on into the 1980s. That’s what the so-called canon wars were about: Just which are the sacred texts for which we are responsible.
But it is also clear that critics have been moving every closer to the ethnographic role as the canon has opened up, as film and other media have come under study, as popular culture in general has become part of the critic’s portfolio. It is not so difficult to think of Hamlet, Wuthering Heights, or even Gravity’s Rainbow, for example, as “sacred” texts, but can we really do that with pulp fiction and romance novels? No, not really. What we can do, however, is interrogate such material in the name of critique.
Thus, for example, Tara McPherson can “read” the 1960s and see a deep resonance between the invention of the UNIX operating system and the restructuring of race relations. As I have recently argued:
She cannot identify any causal mechanism within that world that is capable of producing that resemblance. She is thus in effect positing some transcendental force outside that world that is producing that effect. 
That transcendental force is, of course, unnamed. There’s no mention of any transcendental realm at all. And so we’re left with a resemblance and a mystery.
And that mystery is all it takes to preserve the priestly rhetorical function of the critical enterprise. That’s what the informal norms are about. That’s what became solidified in the 1970s and what remains in place now. And that’s why literary critics do not want to examine form in a robustly descriptive way, nor stray into the computational aspect of cognitive science, for then you’re looking behind the curtain at the mechanisms that keep the whole show running. That’s what ethnographers do. But not priests. Priests keep those things secret.
Is there any possibility of explicitly reorganizing the critical enterprise, at least in part, around the role of ethnographer, the outsider who seeks to understand the basic mechanisms of social and psychological functioning?
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 It’s the theme of a working paper from last year: Prospects: The Limits of Discursive Thinking and the Future of Literary Criticism, November 2015, 72 pp., URL: https://www.academia.edu/17168348/Prospects_The_Limits_of_Discursive_Thinking_and_the_Future_of_Literary_Criticism
It’s also the theme of a recent post, How Do We Understand Literary Criticism?, New Savanna, October 28, 2015, URL: http://new-savanna.blogspot.com/2016/10/how-do-we-understand-literary-criticism.html
 Once More, Why is Literary Form All But Invisible to Literary Criticism? (With a little help from Foucault), New Savanna, October 24, 2015, URL: http://new-savanna.blogspot.com/2016/10/once-more-why-is-literary-form-all-but.html
 A theme I broached some years ago in a post at The Valve, Discipline: The Aesthetic and the Ethnographic, February 7, 2007, URL: http://www.thevalve.org/go/valve/article/discipline_the_aesthetic_and_the_ethnographic/
 Yes, I know, questionable usage. But I prefer it to such awkward creatures as “he or she”.
 Transcendental Critique? The peculiar case of UNIX and race in America in the 1960s, New Savanna, December 30, 2016, URL: http://new-savanna.blogspot.com/2016/12/transcendental-critique-peculiar-case.html