thisBecause you are poor.
In the period most armies were made of levy farmers which tended to be poor
most of there weapons would be farm tools and a shield would be given to them by the local lord.
if the pesant could afford it his first purchace of protection would be a helmet since the shield is gonna protect your body but you would have nothing on your head.
if however you were a richer noble you would be able to buy the best armour money could buy for 2 reasons
1: to protect yourself well and not die
2: to look fashinaoble and say to people look at my wealth (look at amours of kings and you will see what i mean)
there is no reason that someone who could afford amour wouldnt buy said amour and use it in battle since as said above your not hindered movemnet wise and it offers great protection all round






