You could never do permanent stat gains that are only obtainable in current heroic Garrosh for example. You'd never be able to catchup for new players. You'd basically be making content only available to a smaller and smaller audience. I don't know how many people are still playing that cleared vanilla Naxx but it has to be tiny. Can you imagine them balancing stuff around all that collected stat boosts?
And then people would be selling their accounts because they have bonus stats. That's why the only permanent rewards can only be vanity rewards. It'd be an absolute disaster if your character became more powerful just because you played for a long time.