Cataclysm changed so much, it doesn't feel "Classic". So we don't need to re-do that. And as so many folks have noted, Wrath feels like the ending of the story that began in Warcraft III, so it would feel odd to end at TBC.
Talent trees still existed, old gear and recipes weren't changed, all the original quests & zones were still in the game. Wrath is a good place to end Classic Servers, hoping Blizzard keeps all 3 up and running, so that I can hop on to which ever experience I feel like having that day.