The whole argument behind this ruling is that at first the suspect talked and then stopped answering questions. Had the suspect not talked in the first place, then the 5th wouldn't have been voided, but since he did, it was.
I imagine the state arguing it to the jury similar to this: "Mr. Doe was helpful and cooperative at first and happily answered our questions. Then, when we asked him about <insert crime here> he stopped being cooperative and refused to answer any more of our questions. If he wasn't involved with <the crime>, why wouldn't he continue to cooperate?"