Purrloin has actually been seen standing on its hind legs several times before the release of X and Y like in this scene from the anime.
Perhaps Game Freak felt that Purrloin looked better standing on its hind legs, or they did it just to mix things up a bit. Also, storage limitations are out of the question, as 3DS game cards can hold 8 GB of data and X and Y are 1.7 GB.
Only Game Freak can give a decisive answer. This is the most accurate answer I can give.