What is the primary reason why convection occurs in the atmosphere?

The answer is it’s because there is unequal heating of Earth's surface. Actually the convection occurs in the troposphere but not in stratosphere because lower altitudes of the troposphere are warmer than higher altitudes, unlike in the stratosphere. Troposphere temperature declines with altitude in the troposphere but increases with altitude in the stratosphere.

radiative zone

explanation    

radiative zone is the layer of the sun which energy is transferred away from the core by radiation. although, the core is the only part of the sun that produces an appreciable amount of heat through fusion. in fact, 99% of the energy produced by the sun takes place within 24% of the sun's radius.