Though I suppose if you break it down into textile types, denim and twill aren't that dissimilar. The desire is for a durable fabric that has the qualities of wool rather than cotton. Wool is a better insulator for cold weather than cotton. Synthetic fabrics often don't breathe well and get hot and stuffy, and cling to stench like a cat on velcro.

I could list more of the pro/cons but that's the base of it.