Carrier's constraint
Many amphibia and reptiles can and do move very fast over short distances. They do it by moving without breathing, which builds up an "oxygen debt". Humans sometimes do this when running very quickly. 100-metre sprinters do not breathe during a race. They collapse and recover afterwards. Many reptiles and amphibians are ambush predators, which fits that kind of life quite well. There is no reptile or amphibian equivalent to dogs or deer, with their huge capacity for running long distances.