Here's why:

* Fluids are incompressible: This means their volume doesn't change significantly under pressure. This is crucial for Pascal's Principle to work.

* Liquids transmit pressure equally: A change in pressure at any point in a confined liquid is transmitted equally to every point in the liquid. This is the essence of Pascal's Principle.

While gases can also be considered fluids, they are highly compressible and don't behave as consistently under pressure as liquids. This makes them less relevant to the applications of Pascal's Principle.