Thats a heck of a question, and I can't for the life of me come up with a reasonable answer. America's water supply is the envy of the universe..
That must be why all those people from other planets are living in California. ;^)
In some municipalities, the water supply has residual minerals in it that make it taste funny.
Where I live, the water is some of the best in the country. (OK. So I'm spoiled!) Anyway, if DH and I go visit his family in S. Florida, I can't stand the taste of the water. The same holds true if I drink non-filtered water where my Sister lives.