Here's why:

* Charcoal is primarily carbon. Carbon is a non-polar molecule, meaning it doesn't have a positive or negative end.

* Water is a polar molecule. It has a positive and negative end due to the arrangement of its hydrogen and oxygen atoms.

* Like dissolves like. Polar substances dissolve in polar solvents, and non-polar substances dissolve in non-polar solvents. Since charcoal is non-polar and water is polar, they don't mix well.

Although charcoal powder doesn't dissolve, it can still be suspended in water to form a dark mixture.