ACA puts a cap on the profit margins of insurers. It does not put a cap on the total profit of insurers. Mandatory minimum medical loss ratios (MLR) actually could have the opposite of the desired effect. To maintain $X in profits that they enjoyed before the minimum MLR, an insurance company would just need to increase total premiums and benefits. This means that they're less incentivized to reduce benefits/premiums, since their profits rise when both rise. It also more strongly encourages mergers of insurance companies, reducing competition.