Suppose someone absolutely hates / has an agenda to make sure a player ranks as lowly as possible. We're discussing 14 names, with the intention to add only 4.
If we're just submitting lists of 8, the worst this person with an agenda can do is rank their pet peeve player "not top-8". If we're ranking all 14, they can put them 14th, giving every single other player in the round more points than that player.
making it so that the players voters consider to be on the margins for that round are seen as essentially equals, avoids the latter scenario.