New Super Mario Bros. is probably the best possible name they could come up with for what they want to achieve.
Super Mario Bros. DS is too close to Super Mario 64 DS(The Super Mario Advance games don't help with this much either) which was a revamped version of an older Mario title. This creates the connatation that Super Mario Bros. DS is a remake of the original Super Mario Bros. Coming up with a logical subtitle is also hard since there isn't much to work with. We already know the story is the princess is kidnapped by Bowser yet again, there's not much you can do with that to create a clever subtitle. Also, creating a subtitle like Super Mario Bros Double Sized ( As well as the suggested in this thread, Super Mario Bros 2006 Edition) or something of the sort just goes back to feeling like a remake rather than a new game. I've heard people suggest that it should be called Super Mario Bros. Adventure or Super Mario Bros. Legends and this just makes it sound like another Mario RPG game and not a new platformer. Super Mario Bros. Neo has been thrown around by fans and while it's not quite as obnoxious as say Super Mario Bros. Xtreme or Super Mario Bros. Hardcore, it nevertheless doesn't sound very Mario-ish and doesn't fit very well. It'd work well for Mega Man but not Mario.
Ultimately the reason why this name is best for Nintendo is because it's not targeted at you or me. Nintendo knows we'll buy this game no matter what it's called just on virtue of it being a new Mario sidescroller from EAD. Nintendo isn't just releasing software in void for only us traditional gamers anymore. Nintendo's strategy with both the DS and Revolution is about appealing to us, older drop-out gamers and people who never played games much at all before if ever. It's these later two groups who this title is targeted towards. Do you really have any idea how many people played the original Super Mario Bros. but never followed up with any of the subsequent games in the franchise? There a many, many, many people who have because everybody played Super Mario Bros. back then. Your mom played it, your dad played it even your uncle with the funny haircut and beer breath played it. It was a game like Pong and Pac-man before it and Tetris since it's release, managed to penetrate the general populace in ways games like GTA, Madden etc. could only dream of. It's meant to stir memories and emotions in those people, to make them say, "Hey, they made a new Super Mario Bros.! I used to love that game!". Then at the very least, pick it up and look at the back of the box and check it out if not outright buy it.