Irenicus is somewhat overrated of an antagonist
I would assume that most of the appeal of Irenicus is his bad ass voice. Voice actors can elevate characters far above their qualities otherwise.
He's also more of a tragic figure I guess, although his original fall was of his own doing and he ends up just doubling down on his hubris rather than repenting with his subsequent pursuits (and he gets called out for it in SoA's conclusion if I recall correctly). Meanwhile Sarevok is just a guy who wants power for power's sake. Nothing fancy about his motives, but not every piece of fiction needs some complex antagonist. And I think ToB does a good job of fleshing out his character without trying to paint his earlier actions in a different light as some sort of soft retcon. Sarevok's story in ToB is just about him coming to terms with his past failures while acknowledging from the getgo that there's no way to return to the prior status quo as to have another go at it (a.i. fulfilling the Bhaalspawn prophecy in his case). Basically the exact opposite of Irenicus. Funny how the simple villain ends up being the wiser guy in the end, eh?
I think the point of sympathy with Sarevok is supposed to be his birth/raising. In BG1 this is explicitly compared to the MC, who was also "of tainted seed", but was raised in a much more wholesome environment. It's basically nature vs nurture encapsulated. So Sarevok was put in a shitty situation and turns out to be an asshole as a result.
If anything I find Sarevok far more relatable than Irenicus. Possibly the issue with Irenicus is he does all this horrible shit to you and never seems aware of his complete lack of justification, and you only find out about his "tragic backstory" (which isn't even that tragic) at the very last part of the game. So by then your view of him is already cemented.
Also from a ludic perspective, Irenicus's role in the game is a lot of cutscenes that distract the player with no ability to have agency against it. The only "agency robbing" cutscene Sarevok has is the first one that is basically the intro anyway, so Sarevok has a lot less player ill-will built up towards him based on him dicking you over in cutscenes where you can't do anything. Irenicus's style of storytelling is much more emblematic of where the industry eventually went, I suppose.
Reminds me of joking with a friend about Mass Effect 3's storyline when it came out. It instantly jacks you onto the railroad and doesn't let you off. This pisses people off. Not being allowed agency in a videogame, or the illusion of agency, is highly insulting and will make people hate a narrative more than they would otherwise regardless of the quality of the writing.
Hell, now that I think about it... even the ending. Look at how Irenicus is handled. YOU don't kill him, he is just teleported to the Abyss, where he "suffers" the fate he deserves... but really he's still "alive" and it feels like a complete waste of your time, like you never did anything to him. Meanwhile Sarevok is just another NPC after the intro cinematic, and the fight with him canonically kills him with no going back, as if he's just another person in the setting (good!), this only being later revoked somewhat by the ToB expansion mod that let you revive him. This is classic "you're just here to play a role, puppet player, now be a good boy and watch our plans unfold" writing that I figure we mostly experienced first as a JRPG trope, but became cemented in western gaming in the later console eras, and which a lot of people (myself included) completely despise.