The main difference is the type of microbes that lives in each area. The scalp has a higher propensity for yeast and fungi whereas the skin and other locations have a higher level of bacteria than yeast.
Hair follicles have their own microbiome. The follicles push out sebum (the natural oils that make your hair greasy) – which the bacteria and fungus on your scalp will feed on. This can layer up and cause deep-rooted scalp irritation.