Because You can catch lot of Pokemon in Kalos. Kalos Pokedex has 457 Pokemon. It is the largest amount of Pokedex. Probably Gamefreak wanted to introduce a region where we can catch a lot of Pokemon from other generation. If Kalos introduced more Pokemon, then the amount of Pokemon will be increased, and it woild be hard to complete Kalos dex and recieve Shiny Charm. In Gen 6, we were able to get shinies in lot of methods. Horde, Poke Radar, Freind Safari, new three ways to get Shiny. And shiny charm increases the shiny rate, probably Gamefreak didn't want that it would be too hard to recieve shiny charm.
Just GameFreak logic. They wanted it..