Because Jayce and Panth generally don’t or shouldn’t use Duskblade as part of their bread-and-butter kits because they fight in the open.
Jayce’s highest winrate build on op.gg is Ghostblade, Cleaver, Guardian Angel. Even in the jungle, Pantheon’s highest winrate build is Ghostblade, Cleaver, Deadman’s Plate. These champions want early lethality to snowball early, the stat mix from Cleaver to carry the mid-game, and armor and surviving or chasing power to dive turrets as the game goes later.
Zed and Talon took Duskblade because they are designed to jump out of fog and blow enemies up, using their burst and speed to avoid damage. Jayce and Pantheon aren’t.