I agree with Nyrael. If Paradox thought it would be profitable to make an expansion, they would do so. Business is business.
Perhaps the Sengoku period is too much of a niche market for strategy games (itself a fairly small niche market)? Koei has basically given up on publishing Nobunaga's Ambition in English (where I originally cut my strategy gaming teeth in the 1980s), and the fact that Creative Assembly had some success with Total War: Shogun 2 can at least be partly explained by the fact that they only publish a new Total War game every couple years -- their fans will tend to buy the latest iteration regardless of the geographic and historical setting. (Although, I do recall a number of people mentioning on forums that they'd pass on Shogun 2 due to the lack of variety in units and the "boring" setting, opting instead to wait for the next offering.)
It is a shame, though. At least for those of us that enjoy the period and setting.