My theory about that is that any name put forward would be immediately shouted down as a politically correct move.
But more importantly than that - there simply aren’t enough ‘diverse’ candidates to be considered, although I know quite a few people who would be happy to see Dame Shirley Bassey gracing the back of a £20 note.
Alan Turing seems such a good choice, Emily Bronte appeals to me for a variety of reasons but it's also her bicentenary next year so seems apt and Stephen Hawking ( although he not dead obviously) would be a cracking choice. If you want to be a bit more contentious you could have Princess Diana, Oliver Cromwell and Guy Fawkes :)