Neither did I. Always looked at it as a copy cat of Stormbringer.
It's not a copy of Stormbringer (see Blackrazor for that). But it's so un-iconic that there is no agreement over what the sword is like, what it looks like, or whether it is particularly effectual against Vecna or particularly impotent against Vecna. I always saw it as an item that was strictly subordinate thematically to the Vecna regalia, and only was associated with them. So almost by definition I considered it less iconic than the Hand and Eye, which I would also considered less iconic than the Wand of Orcus, Axe of the Dwarven Lords, and Rod of Seven Parts.
My list as I saw it at the beginning
Tier 1:
Wand of Orcus - Most iconic artifact ability, associated with one of the most iconic villains
Axe of the Dwarven Lords - Tolkien reference, PC aspirational artifact, major module written about
Rod of Seven Parts - Major module written about, DM inspirational fluff text, homebrew item (1 piece isn't campaign breaking)
Tier 2:
Hand and Eye of Vecna - Source of endless humor, associated with one of the most iconic villains, various modules written about
Baba Yaga's Dancing Hut - Major module written about
Orbs of Dragonkind - Close cousin appears as major plot element of Dragonlance
Tier 3:
Codex of the Infinite Planes - One of the most powerful items in existence
Cup of Al-Akbar - Major module written about
Heward's Mystical Organ - One of the most powerful items in existence
Machine of Lum the Mad - One of the most powerful items in existence, references in multiple video games
Teeth of Dahlvar-Nar - Appears in a module, very homebrewable (1 tooth is not game breaking)
Tier 4:
Sword of Kas - First +6 sword, PC aspirational item, supporting role in the Vecna myth. However, not that much more powerful than any intelligent sword and a girdle of giant strength (and potentially less powerful), obviously less useful because bigger drawbacks, and harmed by lack of agreement over the item in published text.
Mace of Cuthbert - PC aspirational item
Invulnerable Coat of Arnd - PC aspirational item
Tier 5: All other items appearing in the 1e AD&D DMG
I wouldn't at all be surprised to see different orderings than mine, as obviously this is subjective and depends on the sort of things you've played and encountered ("Die Vecna, Die!"?), but I'm a bit stunned that anyone would rank Sword of Kas above Hand of Vecna. I can imagine picking Hand of Vecna #1, but not the sword.