I'm not sure for the particular reason, but as a programmer, certain games have "closed architecture," meaning the game's code is heavily encrypted (enough so that decryption it is incredibly difficult) or there isn't a clear entry point for creators to modify existing code. Digital rights management systems may also be a roadblock. I don't know what the exact issue is with the Telltale games that makes them unmoddable, but these are a few reasons I knew of. Hope this (somewhat) helps!