History is complicated. The reason the Chinese stopped building (and in fact scuttled) their fleets is collective trauma after the mass genocide carried out by the Mongols. Including the decapitation of the Chinese imperial elite and the erasure and sacking of several notable cities. The reason the Mongols were able to do what no steppe people had done before was because of the centralisation of the hordes under the leadership of Temujin, otherwise known as Genghis Khan.