It has to do with how English adjectives are represented in Klingon. I wouldn't actually have any problem with someone translating that sentence that way in real life. But we had to make a hard decision here on Duolingo about the mistaken assumptions someone might make with that sentence.
In English "dead" is an adjective. In Klingon it is a verb which CANNOT be used adjectivally. Klingon verbs which can be used adjectivally (like tIn "to be big") have "to be" included in the definition since they act directly like verbs and don't use a connecting word in Klingon like they do in English. When you use the English adjective "dead" in a translation like, "the patient is dead," it makes it look like there is a Klingon verb which translates as "to be dead", which looks like a verb that can be used adjectivally. But Hegh cannot be used adjectivally and is instead defined as "to die". Hegh is not a way to describe something, but rather an action that a thing does.
I'm still not completely sure whether this is the best way to handle this or not. As I say, even though the Klingon verb cannot be used adjectivally, using the English adjective is actually a fine way to express the same sentiment. That would argue for us including that translation as a accepted variation. However, I don't want to risk anyone to getting the wrong impression about this verb and we don't really get a chance, when someone uses the translation with the English adjective, to add a note saying, but be careful that you don't think of Hegh as meaning "to be dead". So we have erred on the side of simply rejecting "is dead" as a translation.