Free trade is something that's relatively easy to achieve both with the EU and the rest of the world and another.
Free Movement is something that's been painted a bad so they think nobody will miss it.
He, like most of the politicians now, reckoned people are a lot easier to manipulate if you don't give them too much information.
The government knows we need to get the advantages of the Single Market to avoid economic disaster. But they haven't told the public what those advantages are, hence the "just get on with it".