Here's why:

* Tall Canopy: Rainforests are characterized by incredibly tall trees, forming a dense, multi-layered canopy that blocks out most sunlight.

* Limited Understory: This dense canopy results in a very limited understory, with only shade-tolerant plants able to survive the low light conditions.

While other biomes might have tall trees, the combination of a dense, multi-layered canopy with a limited understory is most characteristic of rainforests.