Tokugawa is basically programmed to be backwards. The basic underlying game mechanic relates to some of the current discussion of a specialist versus a Cottage based economy.
To begin with, if you check Tokugawa's leader head info in the CIVLeaderHeadInfos.XML file, you'll find a glaring backwards tendency, which is his basic refusal to trade techs.
<iTechTradeKnownPercent>100</iTechTradeKnownPercent>
This is actually the highest limit of any of the AI leaders on tech trade. I believe the meaning of it is that he is only willing to trade a tech to a given leader if every other leader known to Tokugawa except the target leader already has it, though it is possible that the value of 100% means that Tokugawa will never trade a tech away.
Compare that to the most notorious tech trader, Mansa Musa, who has a TechTradeKnownPercent value of 0, which I assume means he is willing to trade all techs under general circumstances, including if he is Annoyed with you.
Continuing through the XML file, you find the more important causes to Tokugawa's backwardness, though it is slightly less noticable. These causes work in tandem to lead to his backwards nature.
Tokugawa's two flavors are Military and Science. In fact, in an almost ironic turn, it seems he's more disposed to Science than to Military, though there's a chance I am wrong in this. Anyway, Tokugawa is oriented towards Science, yet not towards commerce generation. He is not a Financial leader, nor is he particularly oriented towards Gold.
It has been proven that a Cottage economy is more powerful than a specialist economy, except under Representation prior to (roughly) Printing Press and/or Free Speech. The only way to utilize Representation so early is by constructing The Pyramids. However, Tokugawa has a low chance of even starting The Pyramids. He is not Industrious. If you check the XML file, you'll notice his random chance to construct a wonder is relatively low.
<iWonderConstructRand>20</iWonderConstructRand>
Compare that to the most wonder driven leader, Louis XIV, who's WonderConstructRand value is 50. Not only that, Tokugawa's favorite civic is Mercantilism, not one of the Government civics, so he has even less inclination to build The Pyramids in order to access his favorite civic earlier. Compare that to Cyrus, for example, who's WonderConstructRand value is 40 (quite high, overall), and who's favorite civic is Representation. Cyrus is very much interested in building The Pyramids in order to run Representation in a specialist based economy.
If you notice when you invade Tokugawa, he tends to have a very high amount of Farms, and far fewer Cottages. This is because he's not interested in accumulating wealth (ie a Gold flavor), yet he is interested in research (ie a Science flavor), and he does so by running a specialist based economy, hence the high number of Farms. He can actually maintain a slightly larger empire than most by accepting a lower science rate through the commerce slider. He makes up for the lower science rate via commerce by running specialists. However, his overall research rate is certainly lower than that of a Cottage based economy because he is not running Representation.
To sum it up, Tokugawa is backwards for the following reasons.
- He prefers to run a specialist economy.
- He is not inclined to build The Pyramids.
- A specialist based economy without Representation is weaker than a Cottage based economy.
- He is antisocial, and he does not like to engage in tech trading.
All of these reasons combine to leave Tokugawa running the poorest type of economy, while not compensating for this by engaging in tech trading.