Because people are dumb.
But really. They’ve had 15 years of theorycrafting trying to maths out the most optimal everything mathematically. Without realizing that no plan survives contact with the enemy. Or, at best, not taking that into account. There’s a mentality that anything less-than-optimum is automatically the worse.
But that’s just not true. There is no RIGHT way to play this game, except, what works. We had a bear MT for MC and BWL, no problems. We had ret paladins who held their own in the DPS, we had shaman DPS that did just fine. Vanilla allows for creative play, and use of your kit. It really gives a good player a lot more leeway than it does in retail where you’re locked into one rotation by design.