Because she's publicly committed herself and us, to leaving the customs union and as the EU have so far not made a single concession on anything, literally nothing, we can't have a soft Brexit?
Unless the EU start to make serious concessions, if we stick to leaving the customs union, I think there will be a no agreement scenario.
Our negotiators just have not made a fist of the fact that we have 40 billion quid that thy want to get their sweaty hands on.