Why can't lawyers promise to win?

Certainly can't promise, because there are many factors that change, and it is possible to lose the lawsuit on a small issue. Lawyers are not omnipotent, and there are other reasons that lead to the failure of litigation. Also, although lawyers know the law, sometimes they can't remember or forget it for a while, which leads to the variable factors becoming immutable factors, and lawyers can't guarantee that they will win the lawsuit.