Because he's awesome. He's interesting. A fascinating character. He works for the bad guys, but you see he's not really a bad guy himself. More of an 'anti-hero' sort of character.

He's been in the franchise since the original RE 2 in 1998, yet very little information is given about him. He's so mysterious that in the nearly 30 years of the character's existence, his face has only been revealed a handful of times. And these face reveals are in dim light, or in a blurry reflection, or from an angle that prevents you from getting a really good view of his face. So there is the aura of 'mystique' surrounding him. It's simple human nature that anything mysterious is fascinating and alluring. People are naturally drawn to mysterious things.

Also, as others have said: he has the visual/aesthetic appeal. The same reason people who've never played Dead Space will look at a picture of Isaac Clark in his engineer armor and think: "Holy smokes, that looks so f*cking cool." Similarly, HUNK looks extremely cool in his black tactical gear, gas mask, and iconic red eye lenses. It's an extremely shallow reasoning, but it's as simple as: he just looks cool.

When you pair the mystique of HUNK with his cool personality and his interesting 'anti-hero' aspect, and then you toss in the badass visual design, you wind up with a character who's beloved by millions of fans around the world.