Dani, try applying a flame to non-cotton fabrics and see what happens. With cotton you get a chance to beat the flames out, with man made fibes (nomex excepted obviously) they will melt and eat into your skin. THe chances of ever being invloved in an aircraft fire are minimal I know but why take the chance. Escpecially as most cottons "breath" far better than man made fibres when you are sweating a cross wind landing in the summer time.