Before the ACA, about 16% of the population in both Missouri and Illinois was uninsured.
Of the uninsured in each state (the blue wedge in the pie above), some are eligible for Medicaid, some are eligible for tax credits to help them pay for insurance and some are not eligible for tax credits. The ineligible group includes those who have access to an employer-sponsored plan, who make more than 400% of the federal poverty level, or who are undocumented immigrants.
Additionally, because Missouri has not expanded Medicaid, some adults fall into the "coverage gap". This includes adults who are not eligible for Medicaid but don't make enough money to be eligible for tax credits.