Because he can easily turn valuable resources into dumb animals
Then 2006's EX Team Rocket Returns combined both Dark Pokemon and Rocket's Pokemon in the same set
For that reason, it fares far better in Starter Deck match-ups and its easy synergies do a lot to push it up the list
I started out with every Nicol Bolas, searched for every card with Bolas in the title, expanded the search to Nicol Bolas quoted in flavor text, and finally checked all the various cards that depict various Nicol Bolas-related things
For example, if a creature that was cast as an Adventure is countered, it's a creature card in the graveyard and the first ability will trigger
Card draw every turn, but just once per turn is reliable here since it only needs artifact dorks to ETB