It is because even if it scale well on the healing part for some champion like Jax, akali or corki, they have WAY more important item to take first.
Akali will rush hextech gunblade so even if DD could be cool on her, she has no reason to get it.
Corki already need to follow a fixed set of item so he can properly get to his huge late game/powerspike, no way DD enter in the dance.
Even if Jax is an item abuser, why take DD when he will ALWAYS build titanic hydra and triforce to have all the powerspike he need and then go tanky? Having sustain is generally for lane issue and he take doran blade for that, I feel he would only take DD for specific case.
So yeah, maybe a build or some changes/buff will make DD mandatory in the future for some of them but atm, it isn't clearly on the radar.