The reason this question is being asked, for the record, is some models will sport fiberglass hulls. While fiberglass is stiffer than polyethylene and therefore more rigid, it is also more brittle upon impact. And, polyethylene itself is not precisely flexible (in the right way).
Polyethylene is the perfect balance of durable, reliable, lightweight, and rigid. Hence, it is why polyethylene hulls are arguably the most preferred by users.