I get why they have different damage types, but it seems to me the role of the short spear in D&D 5e (versatile, throwable) really sets itself up for a fighting style not unlike a quarterstaff. Meaning that anyone proficient in the weapon should be able to effectively use the butt end to strike or smash if the situation calls for it (e.g. skeletons).
I guess it just doesn't make sense to me that the weapon wouldn't be equally effective at either damage type. Has a designer given a reason why it should not be?
I know my table could house rule it, but I don't like tossing rules out until I understand them.