Romney knows that MassCare is completely radioactive, and a real deal breaker for most conservatives. That's why he's not talking about it, nor defending his role in its implementation. He can't take it back, or apologize for it now, because that mea culpa would sink his ship even faster than if he just avoids the topic.
The fact that his judgement and personal integrity failed so badly that he signed that commie legislation into law, is a bridge that he can't cross back over. It's done, and he has to live with the repercussions, whatever they are.