Here's why:

* Abundance: Cellulose makes up the cell walls of plants, and plants are the dominant form of life on Earth. This means that cellulose is found in vast quantities in trees, grasses, and other plant matter.

* Structure: Cellulose is a complex carbohydrate made up of long chains of glucose molecules. It's a very strong and stable molecule, which contributes to its abundance.

* Comparison to other molecules: While other organic molecules like proteins and carbohydrates are also abundant, they don't reach the sheer quantity of cellulose found in the biosphere.

Let me know if you have any other questions!