Another factor that only occured to me later is the Laws of Thermodynamics. Nearly all biological energy on Earth has sunlight as its ultimate source. Ther plants absorb sunlight through photosynthesis, some animals eat the plants for this energy, other animals eat those animals for their energy etc.
Trouble is, the process is inefficient and gets more inefficient the further you go up the food chain. If, for example, the plants only absorb sunlight at 50% efficiency and the rabbits eating the plants only derive 50% efficiency and the foxes that eat the rabbits only derive 50% efficiency, the fox is only getting one-eighth of the potential energy. And some super-predator that ate foxes would get only half of that.
Also, as you work up the food chain, the process becomes more difficult. Grass can absorb sunlight by just sitting there; rabbits need only walk up to the grass; foxes have to chase rabbits and some fox-eating predator would have to chase foxes and kill them, requiring greater strength and speed. For the animals high up in the food chain, just getting a meal can involve a considerable investment in energy. An animal who lives off other predators would have to be an incredibly efficient killer, considering how little energy it gets from each kill.
A food chain containing a plant, insect, frog , snake and hawk can be sustained because the insect, frog and snake are not warm-blooded and thus don’t waste too much of their energy just in temperature regulation.