Tony Vargas
Legend
Because the last edition didn't have that distinction, and everything that edition did was wrongbadfun. ;PWhy do we even have the bludgeoning/piercing/slashing distinction in the rules at all?
Seriously, though, it's another attribute which to distinguish one weapon from another, 5e did away with some from prior editions, like threat/crit, so it added one back in.
IIRC, in the olden days it wasn't a specific attribute of a weapon that it did a type of damage, it was just implicit. Obviously a mace was a bludgeoning weapon, it didn't have a keyword or property or damage type associated with it, it was just mace.
Of course, when you were a kid unfamiliar with the minutiae of pole-arm nomenclature, it was equally obvious that a Lucern Hammer also did bludgeoning damage...