Two reasons are quality control and in-house development but I think these are about to be tested with the 3ds and wii u. For the 3ds specifically we've had games like Pilot Wings made by a third party, Nintendogs which felt rushed or at least sparse on changes, a new Luigi's Mansion on the way also made by a third party, etc.
According to some recent Iwata Asks Nintendo is going to be expanding their partnerships with outside developers and I think Grezzo working on Zelda is an example of this. Its gonna test their quality control on some of their big IPs.
According to some recent Iwata Asks Nintendo is going to be expanding their partnerships with outside developers and I think Grezzo working on Zelda is an example of this. Its gonna test their quality control on some of their big IPs.