We learn that the Qur'ān and the Sunnah form part of the religion. We are told that God has promised to protect the "dhikr" (which I suppose includes both). Then why Allah did not protect the complete Sunnah. Hope you will help in this regards.
By Sunnah we mean the religious practices which the Holy Prophet (sws) instituted as part of the religion under the direct command of the Almighty. The Prophet (sws) made sure that these practices are made a part of the life of the generation of the Companions (sws) who with their consensus transferred them to the next generation which conveyed them to the next and so on. This process of perpetual adherence of the ummah to these practices has remained continuous and uninterrupted. Thus the Sunnah has reached us through the absolutely reliable mode of transfer that is generation to generation mode of transfer and it is secure and authentic beyond a shadow of doubt.
Thus the Sunnah is well defined and clear. There is no doubt regarding its authenticity. In this context the Sunnah does not mean the individual narratives ascribed to the Prophet (sws) recorded in the ḥadīth literature. The ḥadīth basically comprises of the isolated narratives which can at best be called probable truth and remains subject to analysis. It generally consists of the historical narratives regarding the life history of the Prophet, his uswah-e ḥasanah (exemplary conduct) and his interoperation and application of the religious directives contained in the Qur'ān and the Sunnah. It is thus an explanatory source and not the primary one. We need to analyze the Ḥadīth in the light of the Qur'ān, the Sunnah, the established historical fact and the common sense. Thus the Ḥadīth and the Sunnah are two different things. No doubt most of the Ḥadīth report about the way the Prophet performed these sunan but they are not the source of authenticity of the Sunnah which stands proved though an absolutely reliable source of generation to generation mode of transfer.
Example of some of such practices is the prayer, the fasts, the hajj, eating with right hand after invoking Allah's name, saying Alhamdulillah after sneezing, saying assalām-o-alaykum while meeting others and responding with wa alaykum assalām, nikāh and marriage rites, burial rites and other such acts which are the part of the Muslims' life.
There has never been any difference among the Muslims regarding the religious position of these practices. This proves that all the generations of the Muslim ummah has adhered to these practices without failure and all the sects recognize them thus establishing their absolute authenticity.
I hope it helps.