Here's why:

* Passive transport refers to the movement of substances across a cell membrane without requiring energy from the cell. It occurs down the concentration gradient, meaning substances move from an area of high concentration to an area of low concentration.

* Osmosis is a specific type of passive transport where the movement of water molecules occurs across a semi-permeable membrane from a region of high water concentration to a region of low water concentration. This movement is driven by the difference in water potential between the two regions.

Therefore, osmosis is a specialized form of passive transport, focusing on the movement of water specifically.