Because like all rockets it was designed to handle any expected aerodynamic loads with a certain margin with minimal mass.
Essentially the right parts just popped like a balloon. Falling on your side involves much different loads than being propelled into orbit. There's little value to designing a rocket to withstand such a fall.
I assume it's due to the liquid oxygen remaining in the booster; essentially its a towering pressurised container falling on its side causing a breach.
To elaborate, the rocket can take only about 1 atmosphere of pressure in the radial direction, but much more force in the longitudinal direction. It's designed to withstand the stresses of going in a straight line, but as soon as it bends it'll crumple, like a toilet paper cardboard tube.
So when the thing topples over, it bends. And when it bends, it ruptures because it's far in excess of the loads under standard conditions.
Essentially the right parts just popped like a balloon. Falling on your side involves much different loads than being propelled into orbit. There's little value to designing a rocket to withstand such a fall.