They probably really want full save state compatibility. You can actually switch between GB, GB Pocket and GB Color and still have the save state work on all of them because secretly every single GB game are running in GBC mode, but the mode that you choose either uses the GBC color info or outright ignores it and uses different colors.
If you do Super Game Boy support... you start having things that may be incompatible, you can actually have both SGB and GBC modes at the same time together, it's not mutually exclusive. Games can still work like this even though it's definitely not tested to work that way, but they could have just used SGB mode without any other choice like GBC games, and it's probably just that they took the simpler and cheaper choice to avoid confusion from people going "how come I can't play Kirby's Dream Land 2 in Game Boy Pocket mode like I remember playing it".
It can just be dumb like that, I'm sure the developers have looked at these details, but at one point, gotta take a decision.