Because they genuinely thought that they were writing some morally complex narrative where they could slow-burn the reveal that Sylvanas was the bad guy.
Burning Teldrassil was supposed to be questionable, but ultimately justifiable as “okay I guess she went a ~little~ too far, but I’m still loyal”, and then having those transgressions build up little by little until hitting the breaking point when she tries to trick you into killing Saurfang.
But Teldrassil completely broke the narrative because it not only made players uncomfortable with being silently forced to participate in one of the biggest war crimes the game has ever seen, but also played their hand with where Sylvanas was going as a character way too early for what they were planning. And by the time they finally got around to putting in the Saurfang questline, we had already just followed orders for so long that there was no sense of hitting any kind of breaking point.
Even people who wanted to overthrow Sylvanas for months prior were left unsatisfied because of how casually your character just decides to betray the Horde.