Pokémon file format between Gen. 5 and Gen. 6 differs a lot so it wouldn't really be possible to trade between the two.
That and maybe regular DS mode local wireless is simply not compatible with 3DS mode local wireless. I mean we could theoretically be limited only to Wi-Fi, but then we still have the issue of format being different. The Gen. 5 game wouldn't really know how to decode it.
And then you'd still be limited to not being able to bring any Pokémon from Gen. 6 into trade, or any Pokémon with Gen. 6 items or attacks, kinda like Time Capsule back then. But for Gen. 1 and 2 the Pokémon file format was pretty much the same, so they were compatible together for trading. Heck they even had some items in Gen. 2 that were only obtainable from capturing a Pokémon in Gen. 1 and trading it over, and the Pokémon would be holding that item. Game Freak programmed the Gen. 1 games to have some Pokémon be holding items, but you couldn't see them and couldn't use them because held items didn't exist at the time and were only useable once you traded the Pokémon to Gen. 2
How cool is that? :p