The most likely reason the justices ruled as they did in the case of Miranda v. Arizona (1966) is that Miranda was not warned of his right to remain silent.
In this landmark decision, the Supreme Court held that the Fifth Amendment right against self-incrimination requires law enforcement to inform individuals of their rights before interrogation begins. This includes the right to remain silent and the right to an attorney. The Court ruled that without these warnings, any statements made by the suspect during interrogation could not be used as evidence against them in court.