Because her successor, Prince Charles, is still too much of a loose cannon — out of control of the civil service. He has his own advisors, chosen by him , and although paid by the civil service they’re more loyal to him than to the Cabinet Secretary.
The longer that the Queen can be timetabled to do this job and to do that, and the more often that Prince William can be quietly brought onto the scene as a public speaker, opening this or that event and becoming more popular, then when the Queen dies in harness — almost literally ! — the greater are the chances that the royal courtiers will have of swinging the population to jump over Prince Charles and agree that Prince William should be King instead.
The problem for the royal courtiers right now is that although the Queen is still amenable — having been conditioned into dutiful obedience since childhood — Prince William is now kicking up ! He’s opened a couple of public events recently, but he’s more recently let it be known that he has a job of work to do — as a rescue helicopter pilot. He’s sticking with his Dad.