I think its off the back of a new documentary on Saville. There are two this year one by BBC one by Netflix. Apparently Prince William is upset as his mother is depicted in the BBC one.Saville is dead and has been dead for over 10 years. Therefore Charles didn't consult with him at any time recently. Why bring it up now? Probably because the Jubilee is coming up and it makes for a juicy headline.
Note: I'm not defending Charles. I think he's a twat-waffle. But it's not accurate to say he's doing this to ruin the Jubilee.
Not sure if the letters have come to light due to a time bar expiring or an info request or what but it seems like its new information in the public arena. Altgough the attention at the time the original Saville stuff came out was very focused on his relationship with a particular hospital iirc so it may be that scrutiny is just falling to other areas now.
Or perhaps the attention has turned to the Royal relationship because of the Andrew situation.