Here's why:

* Movement against concentration gradient: Active transport moves substances from an area of low concentration to an area of high concentration. This goes against the natural flow of diffusion, which would move substances from high to low concentration.

* Requires energy: To overcome the concentration gradient, cells need to expend energy, usually in the form of ATP (adenosine triphosphate).

Let me know if you'd like to know more about the different types of active transport!