The reason that people prolly haven't answered this post is because they think it is a stupid question, but we were all here at sometime.
You can't emulate a GBA Game on a Slot 1 Card because as soon as you put a Slot 1 Cartridge in, all the extra RAM that is used to run GBA games shuts down. There is no way to activate the RAM again, unless you have a Slot 2 device. If someone were to create a GBA Emu for NDS through Slot 1, it would either not launch at all or have a HUGE lag in gameplay. Thus making a Slot 1 GBA Emu, useless.
The same thing goes for the Wii because as soon as you put in a GCN game, everything that was not in the GCN, shuts down. (Extra processor, Extra RAM, movement with the wiimote, ect) that's why you must use a GCN remote when you play GCN games.
Again, the same thing goes for the new DSi. When you insert a DS Cartridge or a flashcart that forces the DSi into Backwards Compatability mode to play DS ROMs, everything new, shuts down.(Cameras, Extra RAM, Extra Processor, SD Slot, ect)
See a pattern?