Oswald was dead so there was no one to represent in front of the Warren Commission. There was no trial so there was no case to build and no need to provide evidence to a lawyer who had nothing to contribute to the investigation.
That's right. There's simply no provision in our system of justice for a posthumous trial. In our adversarial system, inherited from England, each side presents its best case. Our Constitution provides strong guarantees to a defendant in a criminal trial the right to competent legal counsel of his choice who maintains his confidence and works for his interests; to prepare his case with counsel in private; to confront and question the witnesses against him; to compel evidence and witnesses who can testify in his favor; to testify himself in court in his own defense if he chooses; or to remain silent and not have that silence held against him.
None of these things are possible when the defendant is dead. Oswald did not choose Mark Lane as his counsel. Oswald was not able to confer with Lane in private. Oswald could not provide Lane with a list of friendly witnesses or tell him where to find helpful evidence. Oswald could not help Lane challenge the testimony of the witnesses against him. And of course Oswald could not testify in court in his own defense had he wanted to. So a trial of a dead Oswald in our adversarial system would hardly be fair, now would it?
That's why we don't have posthumous trials, and that's why a commission of inquiry was appointed instead. Although the Warren Commission had subpoena power (granted to them by an act of Congress, subject to the usual Fifth Amendment protections against self-incrimination) they had no power to convict anybody. They were not a court. Their job was to investigate the facts of the assassination as best they could and to present their findings to the President and to the world. They weren't perfect, but I think they did an outstanding job given finite resources and especially the limited time available to them.
The only failing of the Warren Commission as I see it is that they were sometimes too
timid in coming to certain conclusions that seemed obvious. But given the gravity of the subject they were investigating, it was probably best to stay on the conservative side and not arrive at a finding unless the evidence for it was absolutely overwhelming.
Of course, they did find -- because the evidence
was absolutely overwhelming -- that JFK was killed by Lee Harvey Oswald, acting alone. That conclusion has never been
seriously challenged. (Note emphasis on the word
seriously.)