I've advocated that Nintendo annually iterate their hardware to keep the line-up fresh and make some money while enlarging their market by pushing the low-end model down to casuals at a proper price. So far, hasn't happened. :|
I don't think they have the R&D setup to push out a revision every year cell phone style. I also don't know what they could add every year since they cant bump the specs for games and the screen resolution cannot change so a bunch of revisions would end up redundant.