Who said that? I would have just liked to have seen it a few times. Especially on the Glabrazu, who's kinda been the caster type demon. Also I kinda think it would have been neat on the succubus, since I'm a Diablo fan, but their super charm seem nice and dangerous enough in the right situation.Other than this particular complaint of "eldritch blast should have been a given power/spell for most [or all] demons/devils"
I think they were trying to stay close to classic AD&D with most of the monsters, choosing to keep things close to old school. But it's noteworthy that back then ranged capacity in and of itself was a factor in XP value and getting a decent range attack back then was notably harder. Crossbows were long range daggers, bows did just a 1d6, weapon restrictions were pretty strict and casters didn't have the ability to cast all day unless they found a wand of something.I do find it interesting that they took long enough to give the Giant Ape an adequate ranged attack, but missed out on the Tarrasque. That's an intriguing omission, given the relative history of things.