This is politics.
Much better to "reward" her with an "upwards" move whilst simultaneously sneaking someone into place with less or no baggage so the opposition have to start again.
Remember The Peter Principle. There is no hard in rewarding failure as it incentivises all the other losers in the hierarchy.
Usually the Tories wouldn't hesitate at this. It's only because they managed to tie themselves to the concrete anchor that was Brexit that they had no options open.