Question

why is all the water available not fit for human use

Answer 1

Of the 1% of total water available on earth,only 1% of the f.w. is freely available to humans,including animals & plants.The rest is held in aqufiers,which when drawn from below carries varied varieties of salts,which places a strain on kidney & even normal health. 
Majority of rivers in developing countries are heavily polluted & not fit for direct consumption. Such was not the scenario ages back.The population in almost every country around the world has increased - putting a heavy strain on f.w. resources. 
People can make use of water in a better manner & in a civilized way - by charging underground waters - by storing in various check dams,by deeping existing dams during Summer- by recycling used water. 
water has & will become one of the most priced commodity in days to come & one or two oil rich west asian countries are towing ice bergs from Arctic for their use - but after the reverse osmosis technology, this has now stopped.