Why doesn't a negative stain colorize the cells in the smear?

Negative stains don't colorize cells because the cell wall of the bacteria repels the dye. The dyes used for negative stains are acidic and do not penetrate the negatively charged cell walls. Because of this, only the background of the slide will be colored.

The main advantage of using a negative stain over a positive stain is that negative stains can be performed without having to heat fix the sample, allowing the sample to be preserved as much as possible without distortions. By using this procedure, the morphology, arrangement and size of the bacteria are preserved and can be determined more accurately.