Same reason most 3rd parties don't do as well on Nintendo platforms. 1st party is too strong.
The exception to the rule.
don't concede you're correct DR2K even on the DS
New Super Mario Bros. (26.88 million)[2]
Nintendogs All versions (23.26 million)[3]
Mario Kart DS (21.04 million)[2]
Brain Age: Train Your Brain in Minutes a Day! (18.72 million)[3]
Pokémon Diamond and Pearl (17.39 million)[4]
Brain Age 2: More Training in Minutes a Day! (13.71 million)[5]
Pokémon Black and White (12.87 million)[6]
Pokémon HeartGold and SoulSilver (11.90 million)[2]
Animal Crossing: Wild World (10.79 million)[5]
Super Mario 64 DS (9.65 million)[2]
Mario Party DS (8.21 million)[3]
Pokémon Platinum (7.06 million)[3]
Dragon Quest IX: Sentinels of the Starry Skies (5.31 million approximately; 4.29 million in Japan,[7] 1.02 million overseas)[2]
Big Brain Academy (5.01 million)[8]
Pokémon Mystery Dungeon: Explorers of Time and Explorers of Darkness (4.5 million)[5]
Mario and Luigi: Bowser's Inside Story (4.13 million)[2]
The Legend of Zelda: Phantom Hourglass (4.13 million)[8]
Tomodachi Collection (3.2 million)[9]
Professor Layton and the Curious Village (3.17 million)[3]
top 20 games on DS, only 3 being non first party titles. other third party titles find success only because the DS has a fucking HUGE user base. its like a giant eating a giant meal, even the scraps to a normal sized person are sizable.