1. Cause of death isn't necessarily a condition.  (e.g. "blunt force trauma")
2. Cause of death is a point-in-time assertion and there's a need to identify when and who is making the assertion a particular factor was the cause of death.  Asserting a Condition is a cause of death is a distinct action from asserting the patient has a condition
3. Need to know a patient's conditions and need to know that one (or more) of them is asserted as cause of death are separate and it should be possible to share one and not the other.

Cause of Death is best represented as an Observation.  (If you want an extension to link the Observation to the Condition, that'd be fine.)