Because it breathes.
Its a bonus that its (a) cheap and (b) fairly strong, but most importantly it transpires moisture. When I built a brooder in the barn out of tyvek, it never condensed water on the walls inside, thoughe the one beside it did just that. Condensing moisture is a recipe for anarobes and molds i.e. the opposite of healthy infrastructure
Check out the summer chicken shelter I built here, out of an old camper frame my neighbor had.
Total Time:5 hours, total cost <$100. It fits 700 chickens no problem (but Turkeys will roost on top)
The final picture is the chickes, enticed and spread out by heat-lamps enjoying their new shelter.