It is an artifact of the legacy of the system.
The Longsword used to mean basically sword in D&D. There were not any scimitars or rapiers in the game. Over time more and more swords were added. In 5th edition the definition of a Longsword changed to a hand and a half weapon or what used to be called in D&D a Bastard sword. This is fine and actually this definition of a Longsword is more historically accurate. However it does give a mismatch between flavour and mechanics. In my world the slim, elegant blades that Elves are often portrayed as using are called rapiers.
The Longsword used to mean basically sword in D&D. There were not any scimitars or rapiers in the game. Over time more and more swords were added. In 5th edition the definition of a Longsword changed to a hand and a half weapon or what used to be called in D&D a Bastard sword. This is fine and actually this definition of a Longsword is more historically accurate. However it does give a mismatch between flavour and mechanics. In my world the slim, elegant blades that Elves are often portrayed as using are called rapiers.