>The word "Snaga" meaning slave was used by an uruk of another >breed of Mordor orc. This may imply that slavery existed in orc >society. In free orc society, given their clannishness it is >easy to envisage captives from a foreign tribe being reduced to >a slave status. However, how the snaga orc was more of a slave >of Sauron than the uruk is difficult to say.

Perhaps because the Snaga are unable to make themselves are useful in combat as they would as slaves in a support form(scouting,farming, etc.).The Uruk was rarer, and much more powerful in combat, so using them as a support duty would be a waste of combat efficentcy.

Just some thoughts, probably wrong too.