That's not a valid comparison. People don't PuG raids to compete against other PuGs for a reward or anything of the sort. PuGs are (marginally) successful because the content is part of the gearing up process, and even then low skill is compensated by bloated requirements to join. PvP doesn't work like that, people don't PvP to get a piece of gear and then never PvP again, like would happen with a raid once you have everything. The purpose of PvP is on the label, it's to compete against others and come out of top of as many as you can. If people would only compete in case they were bribed into it with rewards other than what's already offered, they aren't interested in the competition. 3v3 is dead because players made it so, not because of some imaginary lack of rewards to promote it.
That's because, as much as it triggers some people when I point it out, PvP in World of Warcraft is a side-activity. The game is group-balanced for PvE content, because that's the real meat of the game. Classes are balanced and continuously tweaked so they have a chance at similar performance when doing PvE - and even at that they repeatedly fail, players always find "bests." Just look at any expansion. New PvE gear, new open world areas, new Dungeons, new Raids... while PvP gets an update on iLevel, some tweaks, and rarely some new Arena. When was the last time an expansion had even an equally split amount of new PvE and PvP content? The first time hasn't happened. PvP isn't balanced to this day on WoW, and it definitely wasn't meant to be back on WotLK, that's just part of what you'd expect anyone playing the expansion to know and accept.