But she was born in the UK. Shouldn't that trump everything else?
Sadly not.
The citizenship laws have been changed so that it is okay to remove British citizenship from someone if there is a real possibility they can obtain another country's citizenship.
In practice this normally means via parents or grandparents.
So citizenship is much more likely to be stripped from someone who is a first or second generation immigrant, even if in the latter case the person is born in the UK.
It basically applies disproportionately heavily on people of colour, who are punished not for what they have done, but for the fact their parents were not born here.
To put it another way, Jimmy Savile and Harold Shipman (if both alive today) would not be able to have their citizenship removed as there is no other available nationality for them to apply for.
Shamima Begum, because of her youth, was theoretically able to apply for Bangladeshi citizenship as she was under 21, and that was all the Home Secretary needed to show under the current laws.