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.