If you add a percentage of rice flour or cornstarch to any sort of breading or pancake, you get a much crispier crust than one made with 100% wheat flour. The Vietnamese banh xeo, which is like a crepe made with just rice flour and coconut milk, no egg, comes out extremely crispy, for example. What is the physical reason that these pure starches cook up crispy?
My guess would be a lack of gluten in those flours. Gluten makes dough sticky and dense, so adding flours with little to no gluten might make it less sticky and thus crispier.
After a quick Google search it is a result of a lack of gluten. From what I gather it is not directly due to the lack of gluten - but rather the lack of direct interaction between starches and gluten.
When starch granules are attacked by enzymes present in flour, they release the sugars that yeast feeds on. Starch also reinforces gluten and absorbs water during baking, helping the gluten to contain the pockets of gas produced by the yeast. Source
For me this is a case of correlation is not necessarily causation, as a couple of other sites mentioned that this can observation also varies from cooking method to cooking method (baking v. steaming etc).
the same reason cake flour makes a better roux than AP flour, higher starch to protein ratio
the pure starch, zero protein end of the spectrum appears in velveting How does velveting work?
but you asked why
proteins, when they cook, curdle at lower temperatures, holding onto more water, not crispy, and at higher temperatures, they begin to decompose a Maillard reaction way, getting messy, oily, and burnty, which is not the crispy we're looking for
heat going into starch, beyond the boiling point of water , drives off the water, leaving the starch matrix to cook crispy. that matrix shrinks a bit, but is still full of holes for water to escape through
heat going into protein, beyond the boiling point of water, drives off the water and begins to create a complex, dense mess of aromatic molecules and eventually soot. the outside is much less permeable to escaping water, so we tend to dump in more heat, worsening the dense uncrispy crust problem