because it lacks some of the ions. trace elements and ions from your body will equilibrate with the water so you will lose some of these electrolytes to the water. also distilled water not freshly prepared on the spot will absorb carbon dioxide from the air to make it slightly acidic. when you drink it, it will lower the overall pH of your body
i'm not sure the "distilled" water you buy at the store is truly distilled. there is a difference between tap water, bottled drinking water, and distilled water. most of the "pure" water bought at the stores that can be used for drinking is purified by reverse osmosis, where as distilled water is purified by boiling and condensing vapor water. reverse osmosis is a cheaper process but still leaves ions in the water. this is for improved water quality and taste. true distilled water is mostly used for chemical purposes.
for your case, depending on what the stuff on your mobo is, distilled water might not clean it off because water and that stuff on you mobo might be different polarity. this is where other stuff would be used to clean, but i would try the water first