The most likely reason the justices ruled as they did in the case of Miranda v. Arizona (1966) is:
Miranda was not warned of his right to remain silent.
In this landmark case, the Supreme Court held that the Fifth Amendment privilege against self-incrimination requires law enforcement to inform individuals of their rights to remain silent and to have an attorney present during interrogations. The Court found that without these warnings, statements made by a suspect could not be considered voluntary and thus could not be used as evidence in court. This ruling led to the establishment of what are now known as "Miranda rights."