Well considering GBA is possibly a snes portable, GBA games offer more content and not just a port of a game. Super mario advance series is called that for good reason, MORE CONTENT. Yes is still the same game from snes, but for actually having a different boss in smb2 zone 3 being the robot birdo instead of recycle bosses, adding voice samples, save feature, additional game play such as finding yoshi eggs and coins, is just a improved game. Same goes for the other games, adding voice samples and improved gameplay, sma4 having e-reader levels and those unique items like boomerang and cape make it a bit more of a game and not just a lazy port wth problems.
Same can be said with other SNES to GBA ports like Donkey Kong country, Final fantasy 1-6 (Excluding 3) and Breath of fire. But most people just prefer snes cause of having better quality in colors, sounds, music, graphics, tv resolution. (And probably nostalgia enthusiast) whatever reasons one have for playing one over the other, they exist and have their own purpose.