I'll try to consider it from an evolutionary developmental perspective. The reason you don't see many predators with horns on their heads is because every evolutionary adaptation is a trade-off. It's a zero sum game. If you invest energy and resources making horns, that means you have less for other, more beneficial things such as developing paws, teeth, or muscles. Additionally, sharp teeth and claws make horns redundant. The reason herbivores develop horns is likely due to the fact that their teeth are formed for eating vegetation, and they need their hooves to support their relatively large and unwieldy bodies due to their multiple stomachs. Developing sharp teeth would detract from their ability to feed, and developing claws would detract from their stability and locomotion. Therefore, developing horns on their heads is a viable alternative, since their other appendages cannot be altered for aggression without losing their functionality. Furthermore, the ability to develop horns showcases an individual's evolutionary fitness, due to the fact that diseased or parasitized animals will have less resources to invest towards developing horns. Horns in this case have the added functionality of being a marker of vitality. This causes a positive feedback loop, whereby horns both increase fitness due to their defensive value and due to the fact that females prefer horns due to it being a marker of fitness. Female animals are less likely to develop horns due to their lower inherent aggression and the need to supply resources towards a womb and offspring.