Originally Posted by
uclacolumbiaunc
Can someone explain why these 3 flights all sell Z for cash for same price, OZ for award for same number of miles, but only one of them is available for RUC???

Those are Delta One flights, not first. While a RUC can be used to upgrade on domestic D1, they need D1 upgrade space be available. What confuses me, if there's upgrade space for GUC (on domestic D1 flight), there should be upgrade available for a RUC.
As for not releasing upgrade space, they know they can sell it.