Nintendo's Game Boy Micro might very well have been the new SP. It plays all GBA games, and in essence was what the SP was to the original GBA. It even sported a headphone jack, which is what many consumers were complaining about the SP lacking! The answer lies in compatibility. The standard GB chip has been removed from the micro, removing with it the ability to play standard GB games. A proprietary charger is used, meaning that, if you lose it, GBA/SP/DS chargers will not work with it. It uses another proprietary cable for connectivity, meaning you need a special cable to link micro to micro, or a different cable for micro to SP/GBA. Added in to the mix that there is no such thing as a micro to gamecube cable, it starts to make me wonder why they even bothered with connectivity in the first place.
The micro makes a great system for single player games, but really, the SP is cheaper, not much bigger, can play GB games, and can connect to everything.