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I am assuming it is an evolutionary reason.
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http://www.sciencemag.org/cgi/content/abstract/1139518
"Further analysis revealed that cats were domesticated in the Near East, probably coincident with agricultural village development in the Fertile Crescent. Domestic cats derive from at least five founders from across this region, whose descendants were transported across the world by human assistance."
So yes, if this study is to be believed, domestic cats are the direct descendants of
the African Wildcat, which evolved in arid climates of Africa and the Middle East.
So there might be some kind of evolutionary reason, but I would tend to doubt it. As other's have mentioned, I don't think it's a normative trait of cats that they hate water, and the fact that people think cats hate water is probably due to the way that humans try to bathe their cats (which almost certainly frightens them), like some other posters have said.