Where wills are involved, a lawyer's testimony will further the intent of the now-dead maker of the will. Grand jury testimony will not similarly further the client's intentions. Strong reasons, said the justices, support posthumous privilege. It encourages full and frank communication between attorney and counsel. It preserves family secrets, things no one else needs to know.
Many privileges protect people, in addition to that between lawyer and client.
Any privilege keeps good, and possibly essential, evidence out of court. That is what privileges are for. Nevertheless, said the Supreme Court, the risk of loss of evidence probably is not that great. Clients informed that privilege will not protect secrets after death probably won't tell lawyers anything they want kept secret. That would attack the very heart of the attorney-client relationship.