It's probably just GameFreak not having the chance to fully optimise development for the next Nintendo console. Again.
They just keep on iterating and refining every time. Diamond/Pearl -> Platinum? Refinements galore, plus some crazy use of 3D (Distortion World). Platinum -> HeartGold/SoulSilver? A proper use of the touchscreen and prettier particles. Oh, and much better music samples. HeartGold/SoulSilver -> Black/White? Every single environment is displayed in 3D. Oh, and those wonderful music samples get carried over.
Watch them figure out a way to make the overworld look absolutely stunning and yet, continue to have stereoscopic 3D on in the next set of games. For those who end up getting a 2DS, it's not going to matter eitherway.