Every year became a competition, and it was unsustainable. Companies could spend millions on their presentation only to "lose" and get their reveals drowned out because Sony announced FF7 or Microsoft got Elder Scrolls etc. On all sides there was a rush to be the winner of the year, and it led to more and more CGI trailers of things 5+ years away just for the big reveal moment. I imagine both Sony and Microsoft would prefer to announce individual games as they come throughout the year, so their reveal is the only big gaming news of the week and everyone is talking about it.