It could also be because they had tons of ideas for Sun / Moon that just didn't fit into the technical limitations of the 3DS and hence decided to make a third version on a console that has a lot more horse power to carry out said ideas? My guess is, though, that we won't hear about this officially before E3.
I see why some are angry but I think it's an overreaction. Sun and Moon have been received very postively, saying they are the best Pokémon-games ever so clearly they didn't skimp on those versions. Putting out a third version next year does not make Sun / Moon worse games, there is just coming a better version next year. Think of it like Fifa, you know a better version of it is coming out next year anyway, maybe even on Scorpio but that doesn't make 17 worse.
Also, Dcubed has a point in Pokémon Company trying to pull a fast one, in some regard.