I don't know why they weren't allowed to see the tapes, but Mr. Heidelberg, having apparently watched too many Grade B legal thrillers, got punted off of the jury for engaging in advocacy based on "evidence" that the folks positing we unwilling to submit under oath to the entire jury.
Please note that the indictment against McVeigh and Nichols includes the phrase "and unknown others." Kinda strange for anyone who says that the official government stance is that McVeigh acted alone.