graspee: I don't see why they had to change the name when it was named before the WoW expansion. There's not even any confusion: it's not called simply "Cataclysm" but "Homeworld: Cataclysm".
I wish I hadn't recently spent money on Blizzard stuff now.
Shukaku: Legal issues I'm afraid, forced their hand. Blizzard copyrighted the word "Cataclysm", so it cannot be used anywhere except in Blizzard approved content.
Trademarked. They
trademarked the word. Copyrights are for creative works (a film, a novel, a song, etc.); a preexisting word can't be copyrighted (though I think a made-up word such as
Jedi might be covered by the copyright of the creative work in which it originated).
Also, having a trademark on a word doesn't mean that no one else can use it; just that its use in the same
category in which it's trademarked is restricted in certain ways to anyone other than the holder of the mark.