Because the number of players is not a accurate representation of the quality of an expansion, the same way movies that make the most money (Avatar, Titanic, Avengers Endgame) are not always the best movies and the most popular videogames (Call of Duty, Fortnite) are not always the best games.
People will always argue this because they feel that sub numbers are "evidence" that make their arguments about expansion qualities superior.
It doesn't matter. Legion will always be listed by players as a good expansion and the sub numbers don't exist. But the majority of arguments about Warcraft aren't based on actual evidence other than people's feelings.