there are verses in the Qur'an which can't be understood without the sunnah, there are verses which cannot be fully understood without the sunnah and then there are those verses whose interpretation changes when contextualised in the circumstances of Muhammad's life. in addition, the sunnah is meant to teach us the character of the example of mankind. So an accurate knowledge of the sunnah is very important to a muslim.
yet the hadiths contained mostly fabrications which hadith scholars had to sift through, and even now there are still false, weak, silly and corrupted hadiths in the sittah, meaning hadith reliability is never guaranteed like it is with the Qur'an and there could be hundreds of mistaken hadith under our noses.
so my question is why didn't allah extend his divine protection to the records of muhammad's biographical details like he did with the Qur'an, as an accurate knowledge of them is integral in understanding the Qur'an and becoming a better person