# Why doesn't the ergodic hypothesis hold for most systems?

Is there a physical (intuitive) explanation for why most systems are not ergodic? As my book states, it is a natural assumption that a system is at least quasi-ergodic; it then proceeds to state that this hypothesis is, in fact, false, and that we need a different basis for statistical mechanics. I don't understand why most systems aren't, and how we can prove this.