The reason given in the Telegraph was that his honorary colonelcy means that the Duke of Cambridge will be able to wear his most 'senior' uniform.
Even photos of the 1953 Coronation show that, by then, the RAF 'ceremonial' uniform (high necked tunic, shoulder boards, gold rank braid, sword with belt and busby with plume) had been phased out. RAF No1 HD, even with a sword belt, is rather a bland uniform, regrettably, so Prince William's choice isn't really very surprising.