The major reason is the legendary.
In Black, you catch Reshiram. In White, you catch Zekrom.
In Black 2, you catch Zekrom. In White 2, you catch Reshiram.
If you pick the same color, you get both legendaries. If you swap, you end up with two of the same Pokémon.
Thanks for the info. Looks like I will get White 2 eventually.