Because he is the most beautiful, complex, benevolent character I have ever seen materialized on paper(And in animation). A basic mind will not see this, and will basically hate him; because he is a walking source of despairs and insecurities that sort of personify a persons own insecurities. A "dark" character is easily misconstrued as something low especially when their reasons to be on such a path is more than baffling.
But it really, really Isn't.
As I've said MANY times, you truly have to know where he's coming from on every end in order to truly appreciate him. He is relatable on heights even for me. I'm attracted to the belief that he's been my favorite character since I was 8 years old for the fact that I would later be able to use my carried understandings of him over the years for the very purpose of explaining and interpreting his character from a better side of view.
Of course this will sound pathetic to the audience but Sasuke has saved my life. When I was an idiotic tween, I really would have ended myself had I suddenly not felt such a deep connection with his character, and not felt alone with my turmoils with my family and the personal calamities I was suffering through disorder after disorder. He made me feel human. And safe.
Through much introspection, for some reason evaluating him is very easy. Because I fit in the same context of perception, that he would be most agreeable with.
And it really is such a bitter shame a lot of people look down on such a character as he. I won't get into the "You don't understand him, you would've done MUCH worse had you lived HIS hell", because although it's fabulously true, I can't convince others of this stature otherwise. People will always have a distaste for him, case and point. And as long as they do, I will continue to speak on Sasuke's behalf.
He is divinely written, and I disagree with the purpose of him being a fangirl character. Kishimoto made him to be Naruto's rival, and in the fashion that keenly likable devil. But ultimately, I believe his creation was meant to hold many a perception, from different values of different people. It depends on what it is of him you exactly regard, and in a better case, come to a point of understanding him and not degrading him with petty class-D insults. I can never see reason with anti-Sasuke's for as long as they contradict themselves and use such foul language in attempt to somehow remain coherent, when altruistically they are just biased and predetermined.
I adore Sasuke, and "cool" is an understatement.