The book may have been buried with Marco Polo. Then it would have decayed and no record would be keep. His story would have been passed on to other people but when a story gets passed on, it changes slightly every time it is told, so we can't really know where he went. One of the reasons may be Marco Polo's desendents each handed down the story of Marco Polo's adventure, from generation to generation. I think that the original copy either is hidden, or is lost.
Nobody really knows for sure.
Marco could have lost it in a river and fell apart. I think some or most of Marco Polo's original copy was damaged by weather during the journey. However, because of oral tradition, we can know where he went and most of what he saw. My hypothesis is that either the original copy was stolen or it got ruined in a storm or a flood or fell into the river. I think the reason why we know a little of what he saw and were he went because some people that might have come with him told us, but did not know or tell all of it.