Linked from
The 22 pages that link to Mount St. Helens, each with the reason it gives.
AndesiteBroader topic: Its eruptions produced abundant andesitic and dacitic lava and tephra.
VolcanologyBroader topic: Its eruption demonstrated how flank collapse can drive a lateral blast.
Ring of FireBroader topic: The Cascadia segment of the ring, and its most studied eruption sequence.
Debris avalancheBroader topic: Its 1980 north-flank collapse produced a vast debris avalanche.