The Black & White games came out in 2010, 2 years later the sequel arrives, now they give it like 6 months and announce another whole generation. They are rushing to keep the brand fresh and alive, i wouldn't quarrel if they were announcing a Wii U pokemon title, but as its another handheld they are really pumping them out!
Normally.. When the anime is nearing its end, there is a new Gen.
This isn't anything new, tbh. B/W came out in 2010, but SS/HG came out in 2009. Like one guy already mention, they probably been working on this for a while.
They make a new Gen, throw in a updated version(or remakes) of that Gen to keep you satisfied until the next Gen.
R/B - Yellow
G/S - Crystal
R/S - Emerald, Remakes of R/B
D/P - Platinum, Remakes of HG/SS
B/W - Black and White 2
That's nothing new with Gamefreak and Pokemon.