The issue is that without the caps, the insurance companies would be much more motivated to deny lots & lots of claims. Imagine you're a regular person with a middle-class income, you urgently need treatment and the insurance company algorithm says 'denied' because they're simply saying 'no' to x% of claims that week. What are your options? Litigation? Yes insurance companies have regulators, but the sheer scale of denials would start to overwhelm them. At a huge scale, this would degrade the quality of American healthcare even more. It would become normal for legitimate claims to be denied all the time.

Anyways, this was the explicit reasoning of the ACA drafters when they included the profit cap. Maybe they were wrong, but this was thought about & discussed a great deal at the time