Basically to advertise Black 2 & White 2. Many upcoming episodes will be having many references to things that will be in Black 2 & White 2 (PokeStar Studios, Virbank City & Roxie, the Junior Cup which is based on the Pokemon World Tournament, Humilau City & Marlon, The Kami Trio's Therian Formes, etc.). Remember the anime is meant to advertise the games, and since Black 2 & White 2 are going to be the newest games then they have to find ways to incorporate new things in those games into the anime. Thus why Ash is going to Virbank City to battle Roxie for the Toxic Badge, his 8th badge.
HOWEVER that doesn't mean they aren't going to Opelucid City nor meeting Drayden. Though it has been hinted at before, it has been officially revealed that Ash & co. are going to Opelucid City and, not only that, but Iris will be battling Drayden! 