Electrical System Failures

The break down is due to insulation failure ?

The insulation will fail when the b/d voltage of the insulation is less than the applied voltage ?

Now which side the applied voltage (with reference to earth) is more ? live side or the neutral side?

Simply put, I think, what gets used most wears faster and more frequently. Look at what has more "activity".

The grid system will always fail at the weakest point, and for the transmission and distribution cable system that would be the joints (unless the pole is hit by a car or tree for example, in which case the joints will probably still be the mechanical failure point).

Live side subject to galvanic corrosion? Thermal/fatigue stress due to power surges?

Just t'inkin.

This almost seems to obvious to state, but since the neutral often has an electrical connection to earth, in a non-fault condition neutral has little to no voltage difference to most of its surroundings. While the "live" side has the full potential difference to its surroundings.

In addition to what RedFred has said Voltage can be supplied Line to Line or Line to Neutral. Since the Neutral is the return path for current by breakering the line the circuit is open.