Shema is mentioned by Moishe in the Torah and it is a mitzvah d’oiraisoh according to a lot of Rishoinim.
The origin of Shema came from Yaakov Avinu. So if the origin came from Yaakov, we should be doing it like Yaakov.
NEVERTHELESS we see that Moishe Rabeinu DID NOT say baruch shem and he DID NOT follow like Yaakov said it.
Therefore WE make a pesharah and say it quietly.