How about because it is the Year of our Lord 2022, the last monarch of the British Empire is dead, and humanity generally agrees now that theft through conquest is a bad idea, and Britain realizes that decolonization has to cost it something to mean something, and giving back a shiny rock on the Queen's hat which literally serves no purpose besides being a gaudy symbol of a shameful, spiteful legacy, really isn't much to ask, all things considered?

And also the other jewels stolen from South Africa. Or at the very least, don't keep them on the regalia.