Because the lower down the food chain you go, the more calories energy a food contains.

So imagine a simple food chain: Sun > Grass > Buffalo > Bear.

To make 1kg of Buffalo, you need 10kg of grass.

To make 1kg of bear, you need 10kg of Buffalo.

The bear needs the caloric equivalent of 100kg of grass, whereas the buffalo requires only 10kg because it eats the plant directly.

From a calorie/energy point of view, the further up the food chain you go, the more energy is lost at each stage and the harder it becomes to get enough energy, which reduces your potential to grow your population.

This is why there will always be significantly more buffalo than bears.

In terms of efficient calories, it is best to be a plant and take your energy directly from the sun.

But since animals cannot do that, being a herbivore is the next most efficient way, with the minimum possible energy lost.