Well that pentagon status sort of forced them to release all Pokemon again as
Celever pointed out. Previous gens you were essentially just told to send your Pokemon up through the games, but for XY if you want to use them in competitions you need the Legendaries rereleased. Also I think they're preparing for a generation lock out. It may not happen for the next generation or the one after that, but eventually they're going to stop supporting Pokemon Bank and when they do they're also going to stop allowing you to transfer your Pokemon from Gen V to Gen VI. By having all Pokemon obtainable in Gen VI every player will at least be able to get the normal Pokemon they want.
It doesn't mean there won't be a XY2/Z though, technically they didn't need to make a HGSS since by then every Pokemon was obtainable in someway but they made a HGSS because they wanted to (and knew it would make them tons of money, however you can also tell they had a lot of passion in making the game).