This likely has to do with the way the game is coded.
Rain of Fire, Pandemonium, Thunderstorm, Flooding, Plague, etc. all select appropriate targets and apply their effects one at a time instead of all at once. Because of the way that the 'targeting' works, Immaterial creatures are immune to mass-CC.
Aka
Immaterial blocks Targeting mass CC like Firestorm
but
Immaterial does not block NonTargeting mass CC like Shields or Flooding.
Flooding activation is targeted on a slot-by-slot basis.
Shield effect activation is probably applied as a "When a creature attacks, apply X effect to it Y% of the time" effect.