1) Too many young people didn't insure themselves meaning that the relatively low cost of having them as insured couldn't be used to balance out the higher cost needed for later on in life. This was putting the primary burden of the cost on the older generations.
By forcing everyone to pay the idea was that you could even out the cost.
2) Pre-existing conditions. I.e. the insurance agencies wouldn't cover you if you came into their plan with an existing condition. To give you an example of that from real life.
I had a stage 1 melanoma. They tried to excise it but didn't get everything out and I had to get more complicated operation.
My insurance wouldn't cover it because they claimed I had that coming into the system. I later convinced them otherwise but until that happened the only thing my doctor could offer me was to monitor my melanoma for the year I had to wait until they would cover me again. Imagine that.