What would lead you to believe that God could not protect oral traditions, but was only able to preserve written traditions?
God could, and is able to do anything He wants. Have I written anything to make think that I believe otherwise?
As far as your point is concerned, God only promised to protect His word, the Bible. Do you think that the oral Traditions handed down by the Popes and Magisterium are just as much God's word as His Bible is?
-ksen