Again that was a "maybe" answer.
History, as much I love history, is a bitch to find to real answers.
Europeans developed plate armor in response to changing weapons and techniques for using them that required large, rigid plates over heavy padding for protection (which, of course, led to the development of new weapons and techniques to defeat the plate armors developed, leading to more sophisticated plate armors, etc., etc.).
Part of the reason for this is likely the sheer volume of warfare in Europe through the medieval period, which was more or less constant (although generally on a remarkably small scale as viewed from a modern perspective). With constantly shifting alliances, and numerous enemies and rivals ready to pounce if they saw a big enough advantage, there was an enormous drive for those with resources (which meant those with the armies, such as they were) to develop new weapons and fighting techniques, and new defenses so give them the edge on the battlefield that might make the difference between winning and dieing.
China was politically unified for most of the same time period with limit internal strife and little cause for development of new weapons, techniques, and defenses in the military. The previous era, the Warring States Period, did see huge advances in weapons, combat techniques, and defenses, for exactly the reasons Europe had them later. Europe, however, had time for the advances of the Warring States Period to percolate westwards and form part of the basis of European warfare technology, along the Silk Road trade routes (in the form of the arms and equipment of the caravan guards, if nothing else). (The Silk Road is known to have been operating during Roman times, and the Romans were well aware of the existence of the Kingdom of Ch'in, if a little fuzzy about the exact location and other details.)
