It's not all about that party, though I brought it up on the other thread. Nor is it about the very different circumstances of their private lives. William and Harry will probably always love each other like brothers, but as adults they just don't appear close any more. You never hear about them socialising together or spending spare time together. There are a few sparse exceptions, a shoot in Spain, weddings of mutual friends, and that's it. It's obvious to me that they've grown apart.
I think it all depends on how one gauges closeness, I suppose. For many people, myself included, closeness to a family member isn't calculated at all by how many social events we attend together. I see family members mostly at holidays. However, there's a constant flow of communication going on between us, via texts and phone calls and e-mails. Most adults I know don't hang out with their siblings all the time (of course, many, if not most, don't live in the same city as their siblings). Personally, I haven't had family members at my birthday parties since I was a child - instead, they're more likely to drop by my house with a gift on the week of my birthday or maybe take me out to lunch. In terms of William, Harry, and the rest of the royal family, most of the get-togethers that we know about likely only scratch the surface.