They probably wanted to get a 3DS game out there ASAP...plus, it's still 9 months away, so it's almost 4 years...
Black and White were released in Japan in Fall/Winter 2010. By October it will be 3 years.
But yeah, they really wanted to increase the marketability of the 3DS, I'd assume. I know many people, including me, were refusing to buy it until it got a main series Pokemon game in its library. --- Official Chespin of the Pokemon X boards.
Second gen was three years to the day; fifth will be about a month longer.
You're just comparing it to the two gens that had remakes in them. Fifth gen, like the first and second, did not. --- Creator of Jay's Journey (see quote!) "It's not ten years old! Therefore, it sucks!" - Nostalgia whores everywhere