To avoid being blamed for the damage of a no deal exit. There's no win for him here. Unless there's a clear personal benefit in damaging the country, even he would choose not to damage the country. And there are no votes now in Brexit.
The real and present danger is that he simply cannot make a decision, or just finds it easier to let us slide into a chaotic no deal rather than go and face Parliament with any deal. It may have been another misjudgment by MPs to insist that our sovereign Parliament should have a final say on any deal - but not to have a vote to approve No Deal.
Bottler Boris's No Deal.