Because it DID happen, it's a huge thing, and it should impact things going forward. If you want a continuous story, that's how it works, one event effects the next. If they don't want that, I guess it's fine, but that not what most fan expect.
you are the only one I have heard complain about it. I am sure it could be a factor for some, but I think you might be projecting your own preferences on most fans. Big things happen in the real world all the time, some people dwell on it, and some move on and don't talk about it either because they don't want to think about it or they can't cope with it so avoid/deny. Just because people aren't talking about/dwelling on it, doesn't mean it didn't happen or wasn't important, but not every world building detail is relevant to every story and a good writer/editor knows when it matters to their story and when it doesn't. World building details should enhance a story, not define it or dictate what the story can or cannot be. Sometimes it is a background detail (even a major one) and doesn't fit or isn't needed for the current story being told so stays in the background and doesn't intrude where it's not needed.