Sylvan Primordial is too easy to bust open a game. Hullbreacher and Opposition Agent are not. The former is a powerful threat that puts the player using it far ahead of others, while the latter two are essentially niche answers (one to opposing card draw, the other to opposing tutors). Overly powerful, game-warping threats are banned pretty often; cards that answer things generally are not.
Currently Playing:
Multiplayer EDH Lists (click italics for a link to the thread!)
[Primer] Lord of Tresserhorn - Don't Tell Me What I Can't Do[Primer] Roon of the Hidden Realm - Rhino Blink
5 Color Tribal Guide (Slivers, Atogs, Allies, Spirits)
Also Playing (most decklists can be found on my profile)
MarathGeistKamahlGrenzoBolasThassaGitrog
PiratesZurVial Smasher&ThrasiosYennettJhoira(cEDH)Strix(Pauper)
Legacy: Maverick
Modern:
Melira PodRIP 1/19/15GWHatebears