Partly Because Xbox One Isn't Backwards Compatible
Microsoft has suggested that players will want to keep their Xbox 360 around as a "vibrant" part of the Xbox ecosystem over the next few years, not least because neither retail nor downloadable games are backwards-compatible with the Xbox One. However, they also plan to reveal a "huge" current-gen announcement at E3.
"[XBLA] games will continue to run on your Xbox 360 for as long as your Xbox 360 runs,” Microsoft’s Phil Spencer told Gamespot. “The box is not backward compatible and we think for somebody who invests in a large digital library that you want to keep your [Xbox 360]. Keep that as a vibrant part of the ecosystem.
“That’s why we’ve made sure that your identity, your friends, are constant between both platforms. You don’t have a separate identity on Xbox One or [Xbox 360]; it’s the same, you are you. My friend’s network is the same on both boxes, my achievements are the same. So we think that service component is an important component to keep constant across both devices."
However, beyond keeping your 360 around to dip into Trials Evolution every once in a while, Spencer also promises strong support for the console over the coming months. “I think the [Xbox 360] is going to be very vibrant for many years,” he continued. “In fact, we have a huge, I think it’s huge, [Xbox 360] announcement for E3 that I keep wanting to talk about, but I can’t.
“I think it will surprise people. I loved what we did with Minecraft; bring Minecraft to the box; six million units later it’s kind of a beast. I think there’s a lot of life in the [Xbox 360] for years to come.”
Perhaps another major XBLA exclusive, perhaps, or a PC franchise tailored for the platform? Either way, we'll find out soon (and potentially buy a bigger A/V cabinet).