D&D legacy. It will always make sense for elves to have proficiency with long swords in out-of-the-box D&D because that's a part of how elves were adapted into AD&D from other sources. Individual campaigns and settings may mess with that at leisure, but continuity of lore from one edition core to another is a good thing.
I'm sorry but I have to disagree with this. Just because something has always been does not mean it should always be. Tradition has been a blocker for progress on many fronts.
[Long Sword with both P/S damage
Create a feat that allows a character to finesse it and gain some other abilities such as Iaijutsu, hero may draw and attack in one single action gaining Advantage on that one attack unless the target is also trained in Iaijutsu? also the feat can allow then to fight Katana/Wakizashi ?]
I like the idea about the feat though. The problem is that I play mostly in organized play and homebrew rules don't apply.