Delta is tough for international upgrades. They can be tough for domestic ones too, now that they’re monetizing 58% of their forward cabin premium seats.
Though they’ve gotten more generous with top tier elites the past couple of years offering confirmed upgrade certificates from any fare (which they didn’t used to do, and lagged both United and American with their international elite upgrade benefit as a result), they’re still pretty tough.
Still — there are things you can do to find upgrade seats.