Armour is a compromise
Heavier armour obviously offers better protection, but is more fatiguing to wear from both weight and heat, which in turn restricts mobility. Not so much an individual soldier (who can perform almost any action armoured they could perform unarmoured) But units of heavily armoured soldiers cannot move about a theatre of conflict as easily as lighter armoured troops.
Tactically therefore heavy armour is more favourable when battles can reasonably be expected to have fairly limited duration, and the combatants have some way of "agreeing" a battle field.
How do Orcs fight?
If they prefer hit and run tactics, favouring several skirmishes a day, wearing an enemy down, over one decisive battle, then endurance and manoeuvrablility become more important than absolute protection.
Does practice and training emphasise co-operation or individual prowess? If the latter (and these are Orcs...) how are units co-ordinated on the battle field? Again lighter armour with improved vision and hearing may be better when command and control relies more on everyone being aware of the tactical situation rather than drill and set pieces.
Orcs are often portrayed as attacking in hordes - using overwhelming numbers. Outfitting one thousand knights in armour is one thing, a ten thousand strong horde is another.
Why do Orcs fight like this?
Perhaps they are nomadic (their high breeding rate means they constantly outstrip local resources forcing them to move on?).
Other answers have discussed the issue of carrying armour. Nomads also may have less access to metal and metal working making those types of armour less available to them, although boiled leather can be nearly as tough as steel (although much thicker).
Captured weapons can easily be used, but heavier armours, especially if they have articulated joints, often require some degree of fitting to the user for full mobility and comfort and so are not as useful. Especially if Orcs are on average a significantly different size or body shape to their usual opponents discouraging the use of armour taken as loot.
May be Orcs have a degree of indifference to personal injury. This could be physiological - reduced pain and shock, rapid wound healing, or it could be psychological. Maybe it is dishonourable to show weakness, or perhaps the guaranteed way to Orc heaven is to be slain in battle with the blood of at least ten foes fresh on your weapon. If Orc culture doesn't value an individual Orc all that highly then a good death that brings glory to your family might be more attractive than a long life.
Factors like these might reduce a perceived need for excessive protection.
If you don't have the armour that makes armoured fighting styles appealing, then you are not going to use armoured fighting styles, which in turn discourages the development of armour, even if that is how your enemies fight. "Puny Humans! Hiding in their skins of steel - for all the good it does them! Ha ha ha ha!"