Forcing an insurance company to provide coverage for those with pre-existing health conditions is like forcing a poker player to bet when his opponent turns over a straight flush.
At that point, insurance companies are no longer in the insurance business. They are in the income redistribution business. They will raise the rates on the healthy to pay the health expenses of the sick. How long before the healthy realize that they are being stolen from? Hence the need for the mandate. It's just another tax that will be called mandatory health insurance.