Here's why:

* Sodium-Potassium Pump: This vital cellular mechanism actively pumps sodium ions out of the cell and potassium ions into the cell. This maintains the concentration gradients crucial for many cellular functions.

* Electrochemical Gradient: The high concentration of sodium ions outside the cell contributes to the electrochemical gradient that drives many processes, such as nerve impulse conduction and muscle contraction.

While other cations are present outside cells, such as calcium (Ca2+), sodium is the dominant cation in the extracellular fluid.