The primary side is the side that has most of the components, especially the ICs. The most common soldering technique these days is reflow soldering. It is simply easier to get precision temperature profiles on one side of the board only.
The secondary side can also support components, but
- it’s harder to control the temperature precisely, and
- gravity works against you as the solder paste begins to melt and the components on the underside of the board want to fall off.
Read More: Multilayer PCB