The GameBoy is distinctly different from the (S)NES. It uses an 8080/Z80-ish CPU core instead of the 6502/65816, the audio and graphics hardware have different

The GameBoy is distinctly different from the (S)NES. It uses an 8080/Z80-ish CPU core instead of the 6502/65816, the audio and graphics hardware have different capabilities than the NES, etc. There were a lot of games that were both on the NES and the GB but they were rarely 1:1 ports for these and other reasons.