Almost no one in this debate ever does. Too bad, since the "why" is the basis of every aspect of this debate.
He is making a statement about **when** ""the embryo should be accorded the status of an individual human with rights to care, protection and life.""
He should realize that any attempt to set any precise "when" can only be as principled as setting a height limit at a carnival ride, or setting the legal drinking age at 1 min after midnight on your 18th birthday.
He should also realize that not being able to connect the "when" with the "why" represents a conceptual break that dehumanizes the whole debate. Whether the people involved in this debate have made that fatal break, or simply have no interest in expressing the connection that they maintain in their thoughts, I can't say.