I think maybe it's just because its the central game in the trio GSC just as yellow was in RBY and maybe game freak just wants to release these central games later so as to get more attention for them as, if they are released at the same time, then the popularity of every game will be cut by one-third or even more in some cases but I don't know. We know game freak can do anything anytime they want so I really can't say anything for sure. ;))