curiosity killed the cat

ever wondered why curiosity killed the CAT? and not something else like a dog? fish? elephant?
or why must it be CURIOSITY which killed the cat? and not TOM, DICK, or even HARRY? that said, i must say that i am not a great fan of cats...(i think i prefer fishes more), so even if curiosity wants to kill it , YAY.
the point is, why must phrases be phrased like that?why can't it be something else and not the thing in the phrase? ok i think i am becoming to sound confusing... i mean i can think of a few more... "let the cat out of the bag" (ok, it's cats again...), " out of the blue" (why must it be blue? why not pink? purple?), " beating about the bush" (why must it be the BUSH? isn't the tree much bigger?), and this is an all time fav..."time and tide waits for no man, or woman" (why must it be the tide? it doesn't even rhyme with time...not that i can think of any word which rhymes at the moment...)
so can someone please enlighten me? me mummy always say i like to ask too many questions. maybe i am a kpo, maybe i am just curious...ok, so will i end up like the cat. ho ho ho.