As I've stated on one of the previous threads, we've lost my parents before my siblings were adults.
Even now, 27 and 24 years later respectively, my brother gets pity "Oh, the poor boy, lost his parents so young", this was also often used by my gran to excuse his sometimes very volatile behaviour. And, of course, once my brother sussed that out, he played it to his advantage.
Something neither my sister or I get or ever got (nor did we want that). But we lost our parents, too.
I think this is what is applied to Harry and William here. Harry gets the "Oh, poor little lamb", whereas William gets - to an extent - forgotten about. Despite the fact that he lost his mother under the very same circumstances. And is probably the only person who knows exactly what Harry went through, in the immediate aftermath with the media circus and in the longer run.
It must have been traumatising for both of them, but strangely only one of them seems to be playing the "poor little me"-card.