I'm surprised no one has yet mentioned that it's because he's the polar opposite of Ozymandias, and Ozy was the villian(?).

Ozy is the epitome of "the ends justify the means." He's logical, calculating, and willing to murder countless innocents if it means bringing about a better world.

Rorschach is a moral absolutist. No end ever justifies evil actions; he does have a harsh sense of justice - there's no "reforming" in his playbook. If you've sinned, you get punished, and for him they're biblically just punishments. Sinners get fire, brimstone, pain, and hell.

Ozy could be reasoned with, if anyone had been as smart and capable; Rorschach could not. These two characters were the bookends of the morality scale in the comics.

I think Rorschach is the most relateable character, at least for men. He represents our inner edgelord. He's the only Everyman character: like us, he has no abilities, training, or gadgets. He's unwaveringly convinced of his rightness; his conviction is his only superpower. He's a little like Orson Scott Card's Ender: when he acts, it's with complete commitment to the destruction of his opponent; he doesn't hold back, and that lets him win (most of the time).

I wonder how populer Rorschach is with women readers; I suspect his fanbase consists mostly of men, because Rorschach is testosterone: rage, violence, righteous anger. There's no negotiation, no rational debate, no weighing costs... just action and reponses to the immediate.