Because a MMO that won't evolve can quickly become boring.
The meta worked in 2.0 because we had exactly two tanks (and two healers) for an 8-man party. When a 3rd one was added, the very strict DPS meta nearly kicked PLD out of the loop. Since any tank must be able to tank anything, if you focus on only one metric, you can't really have all tanks as equally efficient. And what will happen when we'll get a 4th tank ? Or a 5th ?
