Muscles are generally of 3 types -- striated , non striated and cardiac . In fact cardiac muscles also have a few striations in them but still because of their unique activity and specific location they are grouped separately. Now coming to the answer, the question must be " Why are non striated muscles involuntary ? " and not vice versa. Anyways , the muscles which are non striated have a single nucleus in them and they are spindle shaped. They are found in the mucosal layers of the intestine and visceral layers of the inner organs. Their action cannot be controlled by us and hence are involuntary and they are also called smooth muscles.
skeletal muscles.. striated muscles contract only voluntarily with the involvement of nervous system CNS. Smooth muscles can contract either by involvement of nervous system or by receiving innervation by endocrine or other hormonal activation.