The short answer is that the Chinese did not need to explore, as at that time they were the leading world economy, and were self sufficient in luxury items as well. They thus had nothing to gain from exploring and trading, as trade along the land based silk route was sufficient for them. In addition they were faced with Mongol pressure on their northern borders. Instead of putting resources towards oceanic exploration they had nothing to gain from, they decided to use their money and effort to deter the mongols. Another factor is the web of Canals that they already established, thus eliminating the need for transport by sea for 'internal' transport

McNeill & McNeill, The Human Web 121-127