Diana's marketing really has been quite stunning. Since she died (which was undeniably tragic) she has morphed from this complex, tricky woman who cut off her friends without warning, was fickle and difficult, who had more than one affair during her unhappy marriage into this absolute saintly human who never put a foot wrong.
Diana did some amazing things in terms of AIDS awareness and modernising the monarchy. But she was a very flawed teenager when she married and the marriage was a disaster from the start. Totally incompatible couple.
Maybe this is the Queen's way of acknowledging that they were wrong in blocking Charles from marrying Camilla in the first place.