Drow High Magic feat mostly, and low level Sorcerer or Warlock or Bard spells.
The Wizard spells are ideal for using with a low Intelligence - Shield, Protection from Evil and Good, Find Familiar, Feather Fall ..... and they don't scale fast enough on an EK to use effectively in combat. Finally you are limited to 2 Fighter Cantrips (until 10th level). On a high Intelligence character that is going to be Truestrike and Bladeward giving you nothing for utility. Putting a multiclass in there allows you to get Truestrike and Green Flame Blade and two more (minor illusion, Resistance ...) on the multiclass, and get Mage Hand or Prestidigitation on the Fighter in addition to Blade Ward.
Moreover Charisma is just a better ability, tieing to the entire social pillar. People on this board complain all the time above fighters being one-dimensional and not being able to contribute in the other pillars. The changes to this subclass encourage this.