So are the rainfall and temperature. The time of picking is important as well as the skill of the pickers' hands. But it's the area in hand-processing the leaf which probably puts the biggest feather in China's cap. The Chinese are absolute masters of the art, bar none. There is no comparison.
Japan is definitely still on the list of countries making a decent green.
But quality tea is hard to find, and often very pricey when you do. In China, there is not only the fine quality, but the variety is as diverse as only the mind can imagine. Watching highly skilled hands of a master roll and manipulate the leaf during the processing is always a treat. Dragon Well, for example, requires ten distinct hand movements as the leaf is transformed into a finished product. The real test of any quality leaf is, after steeping, the leaf should appear just as it looked the moment it was picked.