Theresa may simply believe that the UK public will not be happy to compromise on immigration even if it means losing the single market. No matters how personally she believes in it. She needs to deliver what the UK public wants.
Of course, she could start by trying to explain how bad it would be without the single market, but that line of thinking has been used and abused for month leading up to the referendum and then after, and still now. There is no point adding a layer.
At the end of the day, regardless the circumstances even staying in the EU, she cannot be sure to be able to deliver prosperity. There is no vindication by the number for her at the end of the tunnel. The economy will not boom like China in the 90's, those days are over. Even staying in the EU, the UK is not protected from another 2007 crisis.
So she can't deliver on prosperity but she can deliver on immigration control, and that also happen to be the number 1 concern of her population and so that's simply what she must do rather what she would want to do.