Pull the stick back and you are applying a bending moment to the fuselage - tail down in this case. The top of the fuselage will be in tension, the bottom in compression.

 

Thin skins handle tension well. Under compression they crumple. Try this with a sheet of paper.

 

Without support from the stringers the bottom of the fuselage would crumple under any serious load.

 

This is for a standard semi-monocoque structure - ribs, stringers and a light skin. A full monocoque has a stiffer, stronger skin. Think Mosquito.