Here's why:

* Glucose is too large and polar to passively diffuse across the hydrophobic lipid bilayer of the cell membrane. It needs help to cross.

* Facilitated diffusion uses transport proteins. These proteins act as channels or carriers that bind to glucose molecules and help them move across the membrane.

* The movement of glucose is down its concentration gradient. This means glucose moves from an area of high concentration to an area of low concentration, driven by the difference in concentration.

Important note: While facilitated diffusion is the primary way glucose enters most cells, some cells can also use active transport to take up glucose against its concentration gradient. This requires energy and involves specific protein pumps.