Honestly I think it really boils down to this. I have two close friends who were really good retail players (Top US 50 guilds), at some point they stopped playing retail entirely and focus purely on Wrath. I’ve always wondered why, as retail raids have much more going on, require more effort, are more engaging, and ultimately, they aren’t 15 years old.
I think that they’ve simply gotten too busy with stuff outside of this game, keeping up with retail mythic raiding is pretty big effort if you want to push the highest keys or go for orange parses in Mythic raiding.
They still want to raid in world of warcraft, clear the hardest content, and they do enjoying tryhard parsing in wrath. All of this is infinitely easier to do in WOTLK raids, they don’t even really play outside of the main raid and maybe joining an alt raid on the weekend.