Jeans simply don't tailor well on account of seams and other factors. Slacks can be, but again, you've got to have the material to start with, and today's "skinny" styles don't.
Incidentally: I mentioned Zoot Suits above. They were actually strongly discouraged in the US by the War Production Board, for being overly wasteful of fabric. Wikipedia has a good history of the suit and the Zoot Suit Riots.