Why did so many important inventions made in China in the 5th and 13th centuries remain unknown in Europe?

European medieval travelers dreamed of exploring territories. Countries often sent delegations to Asia. It was also assumed that backward plebeians live in India and China, which can easily be converted into colonies. But when they got to Asia, travelers saw not scattered tribes, but organized states that owned a fleet, weapons, and advanced technologies for that time. Asia developed as a separate world based on traditions and local religion. The country was fenced in by natural obstacles to the development of relations.