It was easier to tell a new player exactly what increasing Might does, because they see increasing Power increases their damage by an amount they're used to.
From a PvE standpoint, how useful is dropping a target's defense by <X>? Is it an obvious thing? Is it actually counted as a group buff? Why? Is it just overpowered? You can't really follow that as clearly, especially from a new player. You need someone going behind the scenes and propping it up on forums doing the math for you. That's not really fun or enticing for people that don't really know the mechanic, which is what GW2 is trying to breach. We had enough Armor Pen with sunder (with or without Recklessness) charts to prove a mechanic like that isn't "fun".
Show Vulnerability to a player now? Oh hey, increases all damage taken by 1% per stack. It makes sense, and it tells you exactly how much it works, on both PvE and PvP targets.