Because the magnetic storage they used at the time was not compatible with the method used for TV, so they shot a camera at their own monitor. It's like taking a screenshot of your screen with a digital camera instead of pressing print screen. The quality is shit.
Oh... I better tell my conspiracy-crazed phsyco of a father that thats the reason and not what he thinks as a 'cover-up'. Sheesh, to think I listened to him rant about it and wasted a perfectly good sunday morning... Never believe what a crazy person says, they always say.