This is definitely true. The gameboy does have memory mapped cartridges to deal with, but they don't hook quite so deeply into the system; they're much more str

This is definitely true. The gameboy does have memory mapped cartridges to deal with, but they don't hook quite so deeply into the system; they're much more straightforward to implement, and for simpler games you can ignore the mapper entirely. The NES is a different beast; nametables _alone_ will give an early NES author headaches, and they're incredibly important to get right, otherwise games won't be able to scroll properly. Even simpler games like Mario and Metroid rely on basic nametable support being there. In this case I think it's the limitations of the NES itself that led to the mappers doing so much more externally; game authors had no other choice if they wanted to pull off advanced tricks. This trend continued with the Super Nintendo; remember the SuperFX chip? It almost turns the Super Nintendo into a dumb terminal at that point, generating entire screens worth of graphics in the gamepak directly.