Here's why:

* Conduction is the transfer of heat through direct contact between molecules. While it does occur in liquids and gases, it's less efficient than convection due to the larger distances between molecules.

* Convection involves the movement of heated fluids (liquids or gases). As a fluid is heated, it becomes less dense and rises, while cooler, denser fluid sinks. This continuous cycle of rising and sinking creates currents that efficiently transfer heat.

* Radiation is the transfer of heat through electromagnetic waves and is less significant in liquids and gases compared to convection.

In summary, convection is the dominant mode of heat transfer in liquids and gases due to the fluid's ability to move and circulate, facilitating efficient heat distribution.