Because each additional damage matters more. It's not a 1 to 1 thing. Being able to one shot more creature types is orders of magnitude more useful.
+1 Agree. The ability to do more with 1 card and increase card efficiency is the reason why card value increases exponentially with the more it does. Thunderstorm doing 1 damage to all creatures required all six copies coming out in order for it to do what 2 Rain of Fires would do, not to mention, giving the enemy more time to heal.
Yes, but except that now you need only 3 thunderstorms, I still think that the cost is too unbalanced. Also I think that the AoE cards are basically to kill small creatures, for big there are things like paradox, sniper, rage potion and so on; the creatures that can be killed with RoF but not with thunderstorm aren't too many. Maybe I'm wrong, but I still want to say that.