Withholding Evidence in a Criminal Case

Tennessee has certain rules that govern discovery in a criminal case. Rule 16 of The Tennessee Rules of Criminal Procedure limits what the District Attorney has to provide a criminal defense attorney. In a civil case , open discovery is allowed , but when your freedom is at stake it is a different matter. A criminal case from Memphis is currently before the United States Supreme Court on withholding evidence. Justice Souter told The State of Tennessee's attorney " I simply cannot follow your argument because I believe you have just made a statement to me that is utterly irrational". It looks like the The Supreme Court is upset about the withholding of evidence , but if the rules of criminal procedure aren't changed it won't make a difference. Why can't we have full discovery in a criminal case ?