Does Theresa May really think she’s going to win in 2020 if she opts for a hard brexit?
The Conservative party has been economically incompetent since 2010, but it’s mostly been at the expense of lower earners and the unemployed, i.e. not the traditional Tory voter base. These people don’t actually matter much from a political perspective because they don’t tend to vote in general elections.
A hard brexit changes the game quite significantly by putting the financial well-being of not only low earners, but also median and above earners at risk. At this point she’s relying entirely on Jeremy Corbyn still being in charge of the opposition to make an economically destructive Tory party seem palatable to voters. Given how many enemies he has and how many years it is until 2020, that’s a pretty big assumption.