This is because:
1) It takes 15% of Conservative MPs to trigger a leadership election, and there are that many of them who want a hard Brexit.
2) She has a majority of 7 (including the DUP, and bearing in mind that Sinn Féin don't vote), and there are more than 7 Conservative MPs who will vote against a hard Brexit. And quite possibly bring the government down over it.
The whole reason for the snap election last year was that she was paralysed by this situation (with a majority of 10), and with an opposition viewed as unelectable she thought that she could get some extra MPs so that she wouldn't be dependent on nearly every single Conservative MP to pass Brexit. It leaving her worse off is, frankly, hilarious, even if I did wish I was watching it from an independent Scotland rather than being stuck on the inside.
This has led to the ridiculous situation of the cabinet fighting between each other over two different Brexit approaches, neither of which is even slightly acceptable to the other, and both of which are unacceptable to Europe. (See here for why).
Frankly, I don't see a way out of this for her. She can't pass a hard Brexit, she can't pass a soft Brexit. The only thing she can do is keep kicking the can in the hope that the horse can be taught to talk.
Original post on Dreamwidth - there are comments there.