Because people don't want to spend extra money for a slot-2 device to get GBA binaries to run natively. Of course, it's the fact that the GBA slot is there that's going to prevent most people from attempting to write an emulator, as no software emulator would reach the level of perfection the GBA slot already provides.
It was different on the GBA, where the hardware needed to play GB/C roms directly was difficult to obtain, making Goomba and Goomba Color useful emulators, but now it's pretty easy to get the $20 EZ-V 3in1 expansion pack to gain full GBA compatibility.