It is my understanding that one criticism made against May's Deal (which has been rejected by the House of Commons twice already, by historical results no less) is that it is too close to remaining in the EU.
I am not aware of the specifics, but I have learned that the UK's soon-to-end standing in the EU was actually very privileged - probably unfairly so, even - and it makes sense that anyone with a modicum of interest in the well being of the UK would attempt to preserve as much as possible of those privileges.
Ironically, that means giving up some of those privileges, including the right to vote on the European Parlament, while still having to subject the UK to some of most significant consequences of being in the EU. Probably worse, at least where trade is concerned.
All around, there is hardly an upside to speak of.