the Xbox is nothing more than a PC, with a more consistent system architecture, running its applications in a much more paralleled processing mode, including a DRM based production line for its video games and software.
Android is now being pushed on next generation devices, clocked high, and even with dual cores.
Minecraft Pocket Edition has already been released. A Full Port, with minimized GUI for tap screen interface input.
The problem lies with in the server's, and their architecture. Most Xbox live games run off of Microsoft dedicated servers, and use the GamerTag system, coupled with MatchMaking, to create lobbies for people to find other players, then join them, and then to Play with them directly, or semi directly (usually through a Player Host).
My assumption is that each Minecraft distribution uses its own networking paradigm, and proprietary server setups, and the simple fact is that is would take too much time- and pose to be much to costly- to wire these all together.
Some high bro developers have have likely done this in the past, notably Final Fantasy 11, however, it did not actually use the Match Making algorithms, but rather, its own line of dedicated MMORPG servers: and here in lies the problem.