Well, actually, I think it was because her "soul" was split or something, which means she had no real "agency" for any of her actions from Warcraft 3 onwards, all the way to Battle for Azeroth. But that was a very weird twist to the story too, which was very poorly received by both her fans or haters liked a lot, the most random and disempowering developments in her character ever. o_O
She no longer seemed as "tough", "independent" or "cool" for most Horde players (remember her fainting scene after she fired her arrow at her "partner), just a ruined plaything for a very unpopular new evil character who died the expansion he was introduced in - and most Alliance players probably would not have felt any sympathy for her as well given everything. Both sides were let down, considerably so.
It would have been much better had they got an "alive" version of the ranger Sylvanas, and had her return to Azeroth to help clean up the messes that she left behind for everyone.