GAMES!... thats the reason. The price is right, power and marketing are standard for a succesful handheld, but it need more games from:
3rd Party Western Publishers are not supporting the platform like the Eastern guys- EA, Bioware, Activision, Epic, Bethesda, Rockstar, Take-Two, THQ and Ubisoft. All of this company are not suporting the 3DS platform with their big core franchises neither they are releasing a lot quality games (or in some cases they are not supporting the console at all!)
Just look at EA, one of the biggest publishers in the World, they have only released subpar rushed dumbed down launch titles (a Sims game, Fifa 12 and Madden 3DS) and Need for speed: The Run; where is Dead Space? where is NBA Jam? where is Mass Effect? where are Battlefield or Crisis or a Medal of Honor game?
Now compare that to a japanese developer like Capcom, with Big games like Monser Hunter TriG (and the soon to be released Monster Hunter 4), Resident Evil Revelations and Mercenaries, Super Street Figher 3DS, Project X Zone, Professor Layton vs. Ace Attorney, Ace attorney 5, Ex Troopers. Now lets do this with Square Enix, we have games like Kingdom Hearts 3D, Dragon Quest Monsters: TW 3D, Bravely Default: Flying Fairy, Heroes of Ruin, Theathrytm Final Fantasy. This developers actually tried to make quality games and they are supporting big time!