Don't have the data, but I'm willing to bet part of the reason is probably because a lot of people that didn't have an insurance plan before Obamacare now were forced to lest they pay the penalty; that's all the ACA really is anyway: a glorified mandate saying one had to pick private insurance from the government's limited marketplace or they'll be fined. Didn't solve dick...but hey, Obama did it so it must be progress!
The change how preexisting conditions are handled was huge. Before Obamacare it was really hard to get and keep health insurance for people with chronic conditions. Unfortunately it seems the current administration wants to go back to these “glory days”