"Unwilling" is a very misleading term to use. It implies that healers were intentionally ignoring Living Dead and letting their tank die on purpose, but that wasn't the issue. There is no indication that the Dark Knight needed to be healed to max HP at all. No visual queues, no telegraphs, no audio cues... Nothing. The same is true for Doom. The game does jack all to teach you that mechanic or tell you when its in effect. You have the debuff icons, but they are microscopic and drowning in a sea of icons on the party list. Yes, a decent player can pick up on that once they've learned what that does, but that quite frankly isn't good enough.
In other words, healers weren't "unwilling" to heal a Dark Knight. Many of them either didn't know they were supposed to heal the Dark Knight, knew about the mechanic but didn't realize the Dark Knight used it, or realized too late and couldn't heal them in time.