Lich King directly pulls at the nostalgia strings of Wc3 fans(the core of WoW’s playerbase).
Death Knights are the most themeatically cool thing you can implement into any fantasy game.
Also, MMORPGs were still popular in early Wrath.
Soon after, the internet evolved and kind of neutered the genre.
What i mean is, early in WoWs life information wasnt so widespread and mainstream. We didnt know the content front to back 6months before the content even exists. We didnt know the best builds right off the bat.
WoW felt amazing because it had a sense of unknown, and a sense of adventure.
The evolution of the internet has eliminated those feelings through no fault of WoW itself.