Actually, it was never the rewards that irked me about the Achievement system. When they added it, I remember dungeon mentality shifting. I would enter as tank or healer and have members of the group insisting on doing specific things to get an Achievement, sometimes bailing on the group if the Achievement was failed even if a boss was downed.
Additionally, it encourages a certain mindset I prefer not to be around - the “look at my achievement” and “we’re not taking you along unless you have this achievement”. Some like it, and Retail is full of it. I’ll be glad to get away from it finally.
The most explanation you’re likely to get is what they said at the BlizzCon panel. (Check out around 34:30 in the video linked here: ***EVERYTHING We Definitively Know about Classic***)
(1) They weren’t added until WotLK, so even if the code is there, the data isn’t.
(2) While it seemed innocuous, it layered on motivations - and vanilla Azeroth is a world that didn’t need those extra motivations.