In my opinion the reason is that it's according to the difference between linguistics and traditional Grammmar, linguistics describes language in all it's aspect without telling people you should say this or not ex:when some asks you "Who are you "you will say "it's me" .But , according to the traditional Grammar,they prescribe language and they have rules that the speaker must follow ex: you should say "It's I " اتمنى أفدتك بأجابة وبالتوفيق
