They did use metal fasteners, just sparingly, because iron was expensive and often of poor quality. Later on even when quality and availability improved, laws w

They did use metal fasteners, just sparingly, because iron was expensive and often of poor quality. Later on even when quality and availability improved, laws were introduced to limit the use of metal fasteners, largely for political and economic reasons. One of the big factors in wood selection in Japan is rot and insect resistance. Japan has very high humidity and buildings were open much of the time, demanding resistance to mold, termites, etc. Another big factor was time. Fast growing species allowed quicker processing, which was helpful considering Japanese buildings have historically not been built to last. Some do last a long time, but most famous Japanese buildings have actually burned down many times, sometimes dozens. Edo (Tokyo) was well known for basically the whole city catching on fire fairly often. They did use hardwoods, too, but where hard wood was necessary or useful, like in certain beams or components that needed hard wearing.