The point being about the rood screen is that it was a pointed spot to place someone, cf Antoine de la Porte, hence the attempts by French researchers to identify who this mystery person is.
As for why there aren't more chief prelates buried there, I don't know, but I do know from Turku Cathedral as a personal example, they stopped burying under the floor after a certain date as it was getting a little crowded, plus there was an unpleasant stench emanating upwards to the congregation. The vaults containing all these lead coffins are accessed via trapdoors, so even with the thickest stone flooring the smell escapes. From wiki, it seems French monarchs were buried at St Denis, so I am guessing the archbishops would be there, too, up until 1793.