Individual states are allowed to set up their own single-payer systems provided they meet the minimum coverage requirements as spelled out under the Affordable Care Act. Vermont is such an example, having enacted the first state-level single-payer health care system in the United States called Green Mountain Care. It is anticipated to go into effect no later than 2017.
The reason why States should not be able to set up their own Obamacare is because in the end the final decision should be up to the individual and not the State. It is the individual that should have the right whether or not to sign up with Obama care, not the State.