Why do Japanese gamers prefer 3DS over PS-Vita?

[Cheebee]

It's not one or two reasons, but a lot of them.

Obviously it's mainly because of the games. 3DS has a sidescroller and a 3D Mario, Mario Kart, Animal Crossing, Pokemon, Monster Hunter, Resident Evil, Layton, plus tons and tons of other important franchises, both big and small. Vita just doesn't have the same sort of heavy hitters.

Also yes, it's cheaper, but imo the games are the bigger reason why 3DS is far more popular.

All the other reasons listed in the OP -portability, Streetpass stuff, multiplayer, memory cards- are added bonuses that together all add up, but aren't the main reasons Japan prefers 3DS over Vita.