What I mean is that there is no biological or cultural reason that orcs need to wear clothing. We wear it for protection from the environment. Certain aboriginal tribes wear next to nothing; their children may have not so much as a stitch.
And orcs enjoy flouting convention, and what better way to differentiate themselves from 'those elves' (who create) than to not wear clothing?
Funny that most posters here think that orcs reproduce sexually, and that orcs might just be 'modest.' Who knew?
Anyway, I'm not even sure why elves wear clothing, but I think it's because Tolkien has elves being much like us, and as we wear clothes (especially if Middle Earth is in the northern hemisphere...).