Thanks! That's a much better explanation!
It doesn't exclude your comment regarding materials - that would be an additional barrier for adoption. But I should point out here that crossbows in Japan were usually made from whale bone, not domestic animal stuff.
I wouldn't be too hard on Japan's three hundred odd year military stagnation. Japan was, on paper, a united nation under a ceremonial-spiritual emperor and a shogun with all the real power. In reality, the shogun and the powerful clans held the country in a fragile balance of power. Besides the Mongol invasions, it was mostly just samurai fighting other samurai using the similar weapons and tactics. In such a situation there was no need for new weapons. Pre-modern peoples usually only adopted new military systems only after their virtues have been taught through defeat. While people often play up the martial aspect of Japanese nobles, they were just as devoted administrators and, like many governments throughout history, were probably conservative when it came to military investment.
I was always fascinated by Japanese military development history - it's a country that illustrates very well that meaningful military evolution requires prolonged warfare and (usually) external threats. Japan, as a relatively isolated country going through lengthy periods of peace and stability, tended to develop in clear leaps when it comes to military history - whenever large conflicts or potential wars arose, they quickly used imported military technology and indigenous military thought to "catch up" and for a short while were on the forefront of military thought - but later stagnated spectacularly.
Classical japanese samurai, developed during lengthy Gempei war, was absolutely state-of-the-art for the time being. But then it not only didn't evolve in a meaningful (from military PoV) way until Sengoku period, but even declined in usefulness, as lengthy stretches of peace focused samurai on dueling/hunting rather than warfare, and samurai tactics, arms and armor evolved accordingly.
A XV century samurai is an excellent hunter and duelist (hence, among other thing, the fascination with "pitting" samurai against other hypothetical warriors), but in conflict with any dominant military of that era, Japanese army would be utterly crushed. By that time, all meaningful armies began to rely on infantry pike formations with supporting fire from drilled troops. That would have decimated a samurai cavalry charge as surely as a machine gun. Yet, starting with Oda Nobunaga, Japan rapidly adopts both tactics (massed drilled infantry) and equipment (plate armor, firearms) of outsiders.
A very good example is Nobunaga's personal set of armor - when I saw it in a museum in a row of armors aligned in a chronological order, I could immediately pinpoint it as his even without mon - instead of traditional beautiful lamellar/laquered centerpiece, it had a european-style "boring but practical" breastplate. See if you can spot the real armor he wore in this picture:
http://www.oocities.org/azuchiwind/oda_nobunaga_3armors.jpg
Same thing happens in XIX century, when Great Powers of the time successfully demonstrated superiority over Japan, and prompted another rapid military modernization.
Does anyone know of any study resource for firaxis's use of stone crossbowmen though?
I don't know the resource they used, but stone crossbow ("ishiyumi") was a thing. It was usually a modification of siege crossbow though, not hand-held. Here's a
book example.
Hand-held crossbows were a thing in early Japan (called Teppo Yumi), but they were largely confined to recreational purposes. They even had some repeating crossbows from China reverse-engineered, but ultimately found no meaningful use for them (remember that the repeating crossbow is primarily a siege defense tool). In pre-samurai era (before XI century), there were some attempts to outfit Japanese soldiers with crossbows, but the rise of samurai prevented that from happening for the reasons outlined in my previous post. The only crossbow-based implements that were left in service were the siege oyumi, but actually it was also largely phased out by XII century.
Here's a lot of text (in Russian) and lots of pictures for various Japanese crossbows:
http://vk.com/wall-40187114_94
To sum it up, all forms of crossbows, including teppo yumi, ishiyumi both handheld and siege, and oyumi should only be used for units of pre-samurai era Japan.