For the most part, upgrades have been tough to get because (1) premium demand has grown while the supply of premium seats hasn’t kept up, and (2) airlines have shifted their merchandising to take less money for those seats from any willing buyer rather than honor frequent flyer upgrades.
However part of the reason why Delta’s upgrade percentage is so low, it seems, is because gate agents at Delta just don’t process upgrades as reliably as they do at other airlines.