It’s all about the screen ratio. The Pocket is 10:9, same as the original Game Boy. This way all Game Boy and Game Boy Color games perfectly fit the screen. The Pocket can play GBA, sure, but GBA was 3:2, so it’s heavily letterboxed on the Pocket. The Game Bub is a 3:2 screen, while using similar underlying tech of FPGA. Basically, the Game Bub is the right landscape layout and right screen ratio to best play GBA games.