I would wonder as well. They are very expensive (even wholesale), relatively fragile (at least compared to clubs, carts, etc.) and probably not worth it to most players at any significant cost. Tom's right in that most players that would want one at even $15 a round would already have their own.
In fact, I have noticed some courses that at one time had cart-mounted GPS systems having gotten rid of them (one quick example is Woodmont here in Atlanta). The upkeep is very expensive, and the additional revenues very low.