I'm surprised you don't remember the reason because it was in the news 24/7. It's because insurance costs 10X for sick people (because they also draw 10X in insurance), those sick people could not afford their insurance, and the ACA gave those uninsured sick people deeply subsidized insurance, raising the average cost to insurance companies of each insured person
sol_remmy seems to be making the case that lots of sick people didn't have coverage before ACA (Which would be a massive plus point for it if true) and that the number of new sick people buying insurance outweighed the number of new young, healthy people entering the pool, driving costs up.
There seems to be a lot of conflicting info on this, but at least Brookings made the case that the expected effect of ACA did actually happen, lots more people started paying for insurance and the average cost paid by individuals therefore went down (even while the average coverage they could recieve went up).
So we seem to be talking about the same thing, just with radically different facts about the same situation.
Is that before or after the sticker price that hospitals give insurance companies?
I'm not sure anyone can get accurate prices, so I'm immediately distrustful of your 10x number.
There is a very real and obvious reason. Thanks to the ("Affordable" Care Act, aka Obamacare), insurance companies have a captive audience of citizens (but interestingly, not illegal aliens) who MUST purchase their insurance product under threat of fine/imprisonment. There is no incentive to lower prices in this arrangement, and the insurance companies are immune from the pressures of a true competitive marketplace. The insurance marketplace is the equivalent of a prison commissary.