Extension of medicaid to cover all poors but probably have like ten times the cost of obamacare, especially with no mandate-esque price-limiting mechanism.
I don't understand how it would cost more to cover less people...
The mandate is designed to reduce the cost of health insurance; healthy young people getting insurance en masse lowers collective risk and makes everyone's payments cheaper. In the PPACA's private insurance model, people still pay for their own insurance, just the poor get it subsidized from the government, so those cost reductions apply to that assistance as well.
Without the mandate or other ways to limit costs, tripling the size of medicaid overnight would have a devastating impact on federal and state budgets, especially in comparison to the more-or-less revenue neutral Obamacare.