The issue is time. The GA's are doing OP UPs because they have an immediate issue. It is easiest and safest to not touch anyone with "special requests" because they in theory went out of their way to have something different. It is much safer to give it to someone who has no preference. Same as bumps; they usually only go with people who have only carry-on. It is quicker and easier that way. It is about the time and least disruption in my opinion. Because Op Ups are not a benefit it is up to DL to do whatever is easiest for them.