I can think of a great reason not to implement QoL changes that came later: because whether they are good for the game or bad is subjective as hell.
Many thousands of players might think that Dungeon Finder was the greatest thing to ever happen to their WoW experience. Personally, I feel it was the beginning of the downfall of the game. Server community, identity and willingness to interact with strangers all went downhill after LFD. You're really never going to get a definitive consensus on what post-TBC changes were good for the game or bad, and even if you did try to run a poll on Warmane, I bet you the vast majority of people wouldn't even see the poll. Fact is, developers need to think of the "casual" player who does not give feedback and how they'll perceive a feature.
Really the safest method is to keep TBC as close to it's original experience as possible, and then carefully tweak it from there. I mean, that's why most of us are here right? Authentic Burning Crusade is a needle in a haystack. Probably the only thing most of us can agree on is that a higher exp rate does nothing to harm the game. We've all done the leveling over and over - it's old. Let's get to Outland at a decent pace, and then enjoy the real reason we want to play.