They live in places where nutrients are hard or almost impossible to extract from the soil because of its acidity. So they've come to rely on getting nitrogen from insects and small animals. In fact, nutrient-rich soil is poisonous to carnivorous plants. But don't worry, either, if they never seem to catch any insects. They can survive, but they'll grow very slowly.
Most carnivorous plants live in bogs and wetlands.
To be called a bog, an area must have sphagnum moss and water of a specific acidity. Bogs are usually fed by rainwater only. No in-flowing streams or underground springs bring in outside nutrients. Many carnivorous plants grow alongside streams, or in other areas that are consistently moist. If the ground does dry out, some plant species become dormant - they hibernate, in effect.