All those beneficial organisms need food to stay alive so they can help plants grow. Just like people, soil organisms won't do what you want them to do if you don't feed them. But in order to maintain the diversity of organisms in the soil, a wide range of food types are needed. Imagine a city without many, many types of foods available. If the only thing you had to eat were french-fries, you'd go looking for a better place to live quite soon.
Soil organisms are like people.
Therefore, a wide diversity of food types is needed, and the more the better. Think about all the diseases that can attack a plant. The exact set of diseases actually depends on the past history of the soil, including the previous plants grown, management practices used, what diseases the neighbors might have, soil type, and climate. Basically, a minimum of 12 species of bacteria were required to control one species of pathogen on cucumber foliage during one summer. That's just one cultivar of plant, just one summer.