Speculation. Just like every other theory. Other companies manage to put out games with more content that run across all platforms that didn't take as long to put together. My sepculation is that there was a grand vision/game here and the marketers won the argument over how to monetize it, combined with the deadline causing cuts to content. There are ways to make the game fit older consoles other than gutting the game. Halo had much more story than Destiny does. I can't think of a single game I owned or rented on the original xbox that had less content than Destiny does.