First of all, there are more horses, more riders and more business in jumping. Eventing is late in comparison to jumping, especially as they revolutionized the international circuit with the Global Champions Tour for example. There is a lot of money to earn in these kind of circuit, which allows riders to earn their living by showing, so horses have more value. This is also a discipline with more sponsors and visibility, indeed covering a show jumping only requires 3-4 cameras, whereas covering a crosscountry requires much more (at least 8 ) so it’s less broadcasted, so less visible and there are less sponsors.
Because of this, a jumping horse has more value than an eventing horse. However, this is not why a very good eventing horse is more rare. An eventing horse is not a dressage or jumping horse that is less good, but a horse who is good at everything. They are real athletes who, luckily for us, are not yet inaccessible!