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The ACA adds minimum coverage guidelines for those plans, and also requires that they are available to folks with preexisting conditions, including pregnancy.

There are legions of stories of insurance from reputable companies (BCBS, etc) that fail to deliver value when they're needed. Theoretically, ACA holds the issuers to standards that guarantee service.

In combination with all of the above, this ends up being more expensive. There are other causes as well.