Here's why:

* Conduction: Heat transfer through direct contact between molecules. This is less effective in gases because molecules are far apart.

* Convection: Heat transfer through the movement of fluids (liquids or gases). In gases, hotter, less dense areas rise, while cooler, denser areas sink, creating a cycle of heat transfer.

* Radiation: Heat transfer through electromagnetic waves. This can occur in gases, but is often less significant than convection.

So, while all three modes of heat transfer can technically occur in gases, convection is the dominant mechanism.