Regular shampoo strips away the oils in your hair and whilst some people may say this is a bad thing, people prefer not to have oily or greasy hair - and that's fine for your head but not your beard.
Quite simply, beard shampoos are not formulated with the harmful chemicals that strip out your natural sebum oils.
Instead, they are designed to soothe any irritated areas, help maintain moisture and allow for ease of grooming.