I'm watching PBS's "Secrets of" miniseries, which says that executioners at the time were trained in hanging, not beheading, and gave the example of Margaret Poore's execution, where it took ten tries for her to be successfully beheaded. According to the documentary, Henry VIII brought in a specially trained executioner from France for Anne Boleyn's execution; why? If protocol at the time was hanging, why break tradition and go to the trouble of bringing in a foreign executioner?
When I went to the Tower of London, the guide said it was because hanging was seen as undignified and humiliating, and it involved a lot of suffering. But beheading was faster and cleaner (or it was supposed to be, despite Margaret Pole's experience). They could do it in a more secluded place, too, away from the sight of the masses, too. So it was employed for certain noble women, such as Anne Boleyn, Margaret Pole, and Mary, Queen of Scots.