GBC carts run at 5V. If you compare the edges of GB/GBC and GBA carts you'll see a cutout on the side. The cutout is there so GBA carts don't trigger the switch

GBC carts run at 5V. If you compare the edges of GB/GBC and GBA carts you'll see a cutout on the side. The cutout is there so GBA carts don't trigger the switch that puts it into 5V mode for use with GBC games. DS lacks the switch and parts to run 5V carts.