I’ll take a (purely opinionated) stab:
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Illidans story is shorter (in terms of exposure) and has been limited to (basically) two wow expansions. He’s had less game exposure and less opportunity to generate overexposure and hostility from players. Sylvanas was a major component of BfA and Shadowlands (both of which dragged on longer than expected and weren’t well received - even beyond her involvement) making it a lot of exposure and tying her to unenjoyable experiences.
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Sylvanas has been involved in a number of events across expansions as a racial leader. Unfortunately that means she’s had plenty of opportunities to have her personality fit the narrative and not the other way around. That’s not to say Illidan has been portrayed consistently (his BC portrayal wasn’t consistent with Warcraft and Legion wasn’t consistent with BC), but a decade of separation makes it less jarring. Sylvanas literally had an inner monologue in a book that contrasted with her own goals. Honestly, if they had made it so that every time Sylvanas died, one of the Val’kyr actually came from the Maw to take control of her body it would’ve made sense. I still don’t understand why the Val’kyr even had to be sacrificed when she died but whatever. Reasons. Even fans of Sylvanas hated the story because it basically blew what little consistency the character had.
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While Illidan’s eyeblast was a bit edgy most folks didn’t really have a strong attachment to that particular chandelier. Our rapport with Akama (at the time of Black Temple) wasn’t really much. Sylvanas’ most recent victims were Anduin (I’m not a fan but he has some I guess), betraying the PC (at least twice), Saurfang, oh and an entire tree of civilians. Those happened since Legion. We can look at Illidans prior transgressions and while I understand there is no statute of limitations on murder, it feels like when you’re in the thousands of years folks consider it.
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Sylvanas’ involvement from Vanilla onward meant she had plenty of opportunities to upset players. Whether it was raising more undead, fighting the Worgen, or just the fact that she became war chief due to Vol’jin dying. It meant many players already had strong negative feelings for the character. Illidan didn’t generate that many enemies.
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Without piling too much on the “BfA and SL didn’t make much sense” train - yeah they didn’t make much sense.
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I hate arguing the “she’s a woman” part. It’s probably true for some folks, her roles didn’t fit the traditional female role of villainy (poisoning) and instead outright fighting and winning. But I don’t want to speculate on how many folks that applies to. I like to believe it’s not many, but i don’t know for certain.
Basically, the deck was stacked against her already. She had many folks who already disliked her. A jarring full heel turn in the prior expac offended her fans. And the resolution didn’t seem to appease anyone - she wasn’t fully herself, she was also stupid - and happened in an expansion that was generally not enjoyable gameplay wise (and was a giant incoherent illogical mess).