I'm not necessarily questioning the decision, I just want to know the logic behind the decision:
Morellonomicon only works on spellcast, and only when the enemy is under 40% HP, and only after the entire, legendary teir item is completed.
Executioner's Calling, by contrast, is on a relatively cheap item, so you can start countering sustain much earlier, and it applies at all HP values AND on both spellcast (of physical spells) and on basic attack. Plus it's one of two options for a fairly core item.
Again, I'm not saying "this is wrong". I just really want to know what the thought process was to arrive here.