The DS's processing power is much greater than that of the GBA making it more difficult to optimize on current PC's. As pc power and speed increases the ability to emulate the DS and the PSP and other systems no doubt will become easier. There are a couple of DS emulators out there but they are far from perfect. It will take time.
As the speed of the game systems approaches the speeds of the current home pc's the time gap between a systems debut and emulation no doubt become longer than we have become accustomed to in the past.