Similar to aggro, having your early creatures picked off, or a boardwipe on later turns, is a large loss of tempo
Then next we have your foe's exiling the top card of their library instead of their regular card flow and also have to cast it this turn so it might not ever hurt them if it's castable and not a reaction like Counterspell
They try to go through it, but find a splash of black bringing The Scarab God and Vraska, Relic Seeker turns the odds against them
Most of the scenarios above leave some amount of wiggle room on mana to deploy cards like Conclave Tribunal or add another two drop to the board
But every leader is a pre-2016 print, so the ranking reflects era as much as rarity - the strongest single takeaway is the era gradient, not any one rarity's top spot
it's just one that, for the most part, will result in slow, methodical turns if you start thinking about the various ways you can win