Assume you were an insurance company executive and you wanted to maximize profit and you were indifferent to the plight of your customers. What would you do?
I'd release new products regularly, while increasing the insurance premiums of old products. Healthy people would just switch to the new product to avoid the price increase, but people with pre-existing conditions wouldn't be eligible so I could price-gouge them.
So you could get insurance against expensive medical conditions - but if you had to claim on that insurance, suddenly your premiums would start rising by 30% a year, and you would have no alternative because of your pre-existing condition.
The companies that didn't engage in this would then look expensive to healthy people and attractive to unhealthy people. And if the healthy (profitable) people leave and unhealthy (subsidized) people join, meaning premiums have to go up further, you could get a chain reaction putting them out of business.