Kokusho is the most famous of the five legendary Dragon Spirits, and its effect scales up well for multiplayer games, too, while the same can't always be said for its counterparts in other colors
Since we don't have any blockers, this means that Sleeper Agent actually says we take three damage a turn (from being attacked) while our opponent takes two
Poison is a fixed version of that - because it turned out that its very easy to simply buff a single creature and kill people in one hit, often in a mere handful of turns
Functional reprints, especially of core cards like these, are of huge value to cube players who stick to playing singleton
While this is not quite the power level of Skullclamp it does have the added perks of doing stuff on its own and not needing extra mana investments to keep it operational
Unsurprisingly, thanks to this combo wincon, both these cards are incredibly popular in Azula decks, leading to sizable market movement