Because this is a sleazy decision which attempts to make nintendo fans buy multiple 3ds.
Had Nintendo announced a completely new console I would have been fine with it despite how recently the 3ds came out. Instead they made an inbetween which already has a major exclusive game. And guess what, a year or two after we buy the new 3ds nintendo will reveal the actual 3ds successor.
If the hardware can't handle the games they want to make they should either use prepherials or wait to launch the games next generation.







