For one, let's face it: the DS isn't the best system in terms of graphics. Sure, we've seen some beautiful 3D games release on the handheld, such as Dragon Quest Monsters: Joker and The Legend of Zelda: Phantom Hourglass, but a 2D approach for Chinatown Wars would mean more manpower could be focused toward gameplay and packing the title with as much content as possible. I'd hate to see a game have less content because visual assets are taking up way too much space on the game cart.
On top of that, the beauty of a stylus-driven control scheme was meant for the 2D approach. Just look at the mock-up above. With some tweaks to the HUD, and perhaps a tap-targeting system (tap your enemies, they get a ring around them which could provide a sort of autoaim for those that need it), could greatly improve upon the strong foundation of the original GTA games.
Finally, if there are less resources dedicated to making as visually polished game, then Rockstar can pack in more features. Features that could include Wi-Fi play for up to four players, including deathmatch modes and the like. Plus, it could allow them to make as big a game world as they possibly can, which is a big thing for GTA fanatics. They want as big a stomping ground as they can get and I certainly don't blame them.
Any way you slice it, GTA: Chinatown Wars is a big title on DS gamers' radar. Personally, I'd rather it be 2D, but we'll all just have to wait and see what Rockstar has to say about it.
