The short answer is because blonde hair is lighter, or less pigmented, than all other hair colors. Less pigmentation means that there is less color saturation in the hair, leaving room for outside substances to stain and color the hair (think of how white sheets stain more easily than brown or black ones). More lightness means that subtle changes in the hair are drastically more visible (think of how a stain on a white sheet is more pronounced than a stain on a brown or black sheet).
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