Same reason why everyone has an iPod and iPhone instead of an Archos. Its all about the marketing. Our group just purchased 8 RPi units, not because we needed them now, but because we will need them in the future, and couldn't guarantee they would still be available.
Although this DS idea sounds interesting. Is there a large developer presence like the RPi, or is it still roll-your-own? Most developers (looking at you, Microsoft) have locked their hardware down so hard, I don't know if anyone will ever make it reusable. The xbox 360 has been out for 8 years, and the only way to free it from its shackles is to find a unit that is 6 years old, has never been connected to the internet, and happens to have a special header on the mainboard. You can then bit-bang it into submission.