The Affordable Care Act promised - for a three year period - that the federal government would compensate insurers who lost money on the exchanges. But Congress refused to appropriate the money. The government found a way to fill the gap by tax credits. Republicans, seeking to kill the ACA, banned compensatory payments to insurers.
That (plus low enrollment by the young who were undeterred by the tax penalty) is why insurers have withdrawn from the exchanges. Another case of Republican blaming Obama for a a failure that they engineered.- gwc
Trouble on the Exchanges — Does the United States Owe Billions to Health Insurers? — NEJM: Perspective from The New England Journal of Medicine