Dolphin's GBA connectivity has stagnated for some time with no major additions in well over half a decade.
In the case of GameCube to Game Boy Advance connectivity, we are incredibly lucky to have had talented developers from both GBA and GameCube circles create our current protocol for supporting 'GBA' controllers. You have to understand two different consoles, how these systems communicate with each other, adapt to latency restrictions, and have expertise across two different projects. This task becomes even more difficult when you consider connecting two different emulators together. Connecting multiple emulators together is a complicated and difficult task.
