Why are there few tetrapod dinosaur carnivores?
Most of the dinosaur predators were derived from a common ancestor. The body plan was efficient, and the theropod predators speciated widely (including birds). But statistically, the number of therapod predators are not independent observations, but linked to each other through ancestry. Similarly, most mammalian predators (Carnivora) are linked to a common ancestor. Both forms worked, but there is not evolutionary pressure to evolve toward a particular form; only pressure to favor successful accidents. So it is an interesting question, but it could be that the predator niche was filled by bipeds in dinosaurs and quadrapeds in mammals.