thanks man :thumb:Because save states save specific information that is specific to the emulator or because the save state is written in a different manner that the other emulator was not programmed to understand.
If the game that you're playing has battery saves (.sav extension, if I remember correctly), then you can transfer those because those are actual saves that the game produces through the emulator that each NES emulator can read. I don't think that many NES games have the battery-backed save though. Every generation of gaming after the NES started supporting battery-backed saves more commonly. Only a few like Zelda had this feature on the NES. The way that you know for sure if a game supports battery-backed saves is if the game has a save selection screen before you start (like Zelda). Then you can take that save file to another emulator.