Hi Kevin, welcome to the site
To answer your question... I believe the main reason would be, that horsebows are not as common as the average longbow or recurve. Here in Europe we've got mainly Grozer and Kassai, who craft and sell many Asiatic style recurves. In USA, I actually don't know of any bowyers who mainly craft and sell these bows. There are custom bowmakers like Lukas Novotny of the Saluki Bow Co., and James Parker of Hunt Worthy Productions, but these custom bows are more expensive and exclusive than what Grozer and Kassai can offer, not to mention the waiting time you have to endure to wait for these custom bows, so they aren't really accesible to everyone.
I, myself, have got a bow from Saluki. I had to wait a year, and pay well over 1000$ in total, as I handpicked all the woods myself. If you let Lukas just craft one using his imagination, it won't cost nearly as much, but it will still be in the upper end of pricey bows.
Do not despair, it is certainly possible to acquire a horsebow without paying this much. Attilas Archery sell some fine horsebows for around half the price of a Saluki horsebow, and don't think that this means, that the quality of these bows equal the cost. This is not the case. But I won't lie to you; there IS a difference in both looks and performance, between the less expensive and more expensive bows. For some, it just ain't worth the difference in cash. That's all up to you