Someone made reference to vulnerability earlier, but didn't explicitly state it.
Prior to
patch 1.9, there used to something called vulnerability bonuses on spells. These were essentially the opposite of partial resists. This caused a number of problems.
Firstly and probably most importantly, vulnerabitly could cause up to 4x damage and had the ability to stack with criticals. So, at the time when shadowbolts were hitting for 8-900, you could literally be seeing hits for over 9000 counting full vulnerability and berzerking buff. Note: the Drakonoids in BWL would buff themselves with an ability that slightly raised their resistance to all schools of magic and reduced their resistance to one school by 400~ish.
Secondly, the stat scaled absurdly very well. Not being a warlock, I can only recall being quoted that this was roughly 17% nerf to Curse of Elements/Shadows (hopefully someone can verify) even counting the +10% damage change to CoE/S. Assuming this is correct, it means that using today's socketing, spell penetration would be roughly 10 times more effective per item point that spell critical strike rating. Without the nerf, it would have been *the* stat to stack, no question.
Basically, post 1.9 patch resistance went from a PvE and PvP spell to a purely PvP spell. It allowed them to slot it into gear more freely (if you look, 1.9 brought in AQ gear, which was much more liberal on spell penetration) to counteract resistance gear that was being used in BGs (remember, all tier1 and 2 and many blues had resistances on them).