Because too clean is bad for you:
Water molecules have a slight negative charge, which means they’re good at dissolving or pulling other molecules apart. When water is in an ultrapure state [such as the kind used in electronics manufacturing], it’s a "super cleaner," sucking out the tiniest specks of dirt and leaving your computer’s brain squeaky clean. But if you were to drink ultra-pure water, it would literally drink you back. The moment it came through your lips, it would start leaching valuable minerals from your saliva.
Update from a chemist:
Water is not negative; it's "polar", with one end negative and the other end positive. It's somewhat analogous to a battery having a positive and a negative terminal. One commenter on the Scientific American site (not me) wrote a nice correction:
"Water molecules have a slight negative charge" On one side (the oxygen atom). On the other side (the hydrogen atoms), they have a slight positive charge. That’s why both positive and negative ions are soluble in water. Of course, it’s also why water exists at all outside of strong electromagnetic fields or a vacuum.
Stripping water down to an ultrapure state makes it unfit for human consumption. You’d need to drink an awful lot of it to notice any effects, though. Tap water is practically pure. It is nothing like blood or lymph or saliva – those are halfway to seawater.
Update from another reader:
Anyone who has ever worked in the field of kidney dialysis is well aware of this fact. Many years ago I took a technical job at a clinic maintaining the dialysis machines. R.O. (Reverse Osmosis) water, made outrageously pure, is one of the primary ingredients in the various chemical cocktails mixed for the treatment. We were warned from the first day of the job to never drink the RO water, as it was so pure it literally leech the minerals right out our bones. We were told if we persisted in the habit we could actually cause a sort of self-imposed osteoporosis as we essentially urinated our bones down the toilet.
The Dish 