Here's why:

* Cells are the fundamental unit of life. They are the smallest units that can carry out all the processes necessary for life.

* Organelles are structures *within* cells that have specific functions (like mitochondria, nucleus, etc.).

* Tissues are groups of similar cells working together.

* Organs are groups of different tissues working together.

* Organ Systems are groups of organs working together.

Therefore, there is no level of organization that is *smaller* than a cell.