I think it might have something to do in part with the difficult position she made for herself by nailing her colours firmly to the Transwomen Are Women mast, despite the fact that the majority in Scotland don’t agree with her.
From a comment on another site
“If you accept that “Transwomen are women” and legislate to that effect, you must also accept that they may attend all female spaces, including female prisons, hospital wards, single sex girls schools and so on. They must also be allowed to play female sports, including rugby and boxing. This would almost certainly be reinforced by the courts if legally transwomen are indeed women.
So we need to remember that when hearing Sturgeon’s (and indeed Starmer’s) answer to that question”.
Most of the folk I know are not concerned about the Gender Recognition Policy as the mess Westminster have made of the Economy.
They are too worried about the huge rise in food, fuel and everyday living expenses.
She is still very popular and well regarded for the way she led us through the pandemic.
The Gender Recognition Act was voted in by 88 for and 39 against.
The "for" votes included the votes of both the Scottish Labour leader and the Scottish Liberal Democrat Leader.
It brings us in alignment with Norway, Denmark, Ireland, Iceland, Malta, Luxembourg and some others who have managed to overcome all the horror stories predicted by some.
Already schools here and presumably elsewhere are dealing with transgender pupils.
I don't think for one minute this is any part of the reason for her resignation.