NO EVIDENCE can be kept from the defense.
Don't capital case defendants get high-priced legal representation? If so, the defense really dropped the ball.
Sure, all evidence is SUPPOSED to be available to the defense. Do you really think that prosecutors always do what they are supposed to do?
Public defenders are the lowest paid lawyers and they are often filled by people who are fresh out of law school and are just minding their time until they can get into something more lucrative.
Even if a public defender has the type of heroic personality who will look under every nook and cranny to ensure his client is defended fully, they just don’t have the funds to do so and they have a few dozen client files sitting on their desks at all times that need their attention.