Limestone filtered water is not unique to Kentucky, arguably the finest limestone in the Country is in Indiana and utilized in structures from the Empire State Building to the National Cathedral in Washington, DC. In addition to being the choice of builders and architects limestone does something else remarkably well, it filters iron out of water. Everybody agrees even a small amount of iron will ruin whisky.
In times past, even over a century ago, distillerys as diverse as Glenmore in Owensboro, Stitzel-Weller in Shively and Jack Daniels in Tennessee shared a common thread. They all had access to Limestone purified water that came out of the ground at a cool 55-56 degree temperature. Would warm river water do? Yes, of course it would so long as it contained the beneficial trace elements (lime, phosphorus) and was iron free.
So other than the fact the water is clean (less likely to infect yeast or taint the spirit) what is the advantage of it being cold?
Well, yeast is a stubborn organism that doesn't like to work in hot weather. Some heat will be generated anyway during the fermenting process and if you don't have the means to cool the vats when the weather heats up then the only option is to close down during the Summer months which is exactly what many distillers did. The answer was to locate your distillery at a place where you had a plentiful supply of already cold water to chill your condensers and equipment.
So it was a combination of history, a distilling heritage brought over from the Old Country, climate and clean, cold, limestone filtered water that made Kentucky the place to be.
If I'm ever present when a tour guide/distiller starts going on about their special water I may well ask what trace elements it contains.