I agree with all the theories here and i'm thinking it's a little bit of everything which has caused things to come to this point.
William's role has changed as HMQ has aged. He'll be concentrating more on his own household. The swirling rumors. The increasing public role. He'll have had less time for Harry.
Harry's role has changed massively over the last few years. The army job, the playboy prince has long gone. Yes he's got married and had a child in a short space of time. Big changes.
And yes, although he witnessed first hand what happened to his mother that doesn't mean he's necessarily picked up the tools to help deal with it or prevent it happening to anyone else. It didn't happen to him, it happened to someone he loved. While that was going on and ever since until now the press and the public were always very pro Harry.
The way H & M have tried to manage their exposure has been quite a mixed bag. I agree they've made mistakes and it's all spiraling a bit now.