Because they are not nice. They are capable of the most cruelty of all the archetypes. Normally, the nice guys only have an outward expression of niceness, but are secretly not nice on the inside

Stefan is considered the "nice guy", while his brother Damon is one of the "bad guys". Damon does some pretty unforgiveable things on the show. He kills Stefan's best friend, he kills humans without a second thought, he treats Elena poorly, and he often abandons the plan and goes rogue in order to achieve his own goals. When he is at his absolute worst, he kills humans and other vampires without mercy, and his actions are so reckless that he risks exposing vampires everywhere.

Stefan, on the other hand, is much kinder...normally. He's the so-called "good" vampire, who doesn't even drink human blood. But the reason he is the nice guy is because he is secretly the Worst Guy. Whenever he does get a taste of human blood, he goes on a frenzy and binges on human blood more than any other character in the show. He's a "ripper", meaning he will devour humans so viciously while feeding that he tears their heads from their bodies in his bloodlust. He is a genuinely cruel person. In his worst state, he makes Damon look like a hero. Stefan ends up going on a killing frenzy so severe that he kills off major characters from the show and lifelong friends of the group. He didn't do it because he was being controlled; he did it because he was a fiend at heart, an utterly contemptuous person who didn't merely lack control of his dark side but fully embraced and nurtured it.

And yet he was the "nice guy". In my estimation, he wasn't nice at all. His niceness was only a mask he wore to hide his true nature. Damon never went as far as Stefan did on the show. Damon was much more "predictable" in his evil; he was merely acting on emotion. Whatever he felt like, he did, because nothing matters in the end. Stefan was being nice as some kind of penance for his sins, because he knew that inside of himself was nothing but evil.

Hulk and Bruce Banner

Let's consider some actual nice guys, like Superman and Captain America. Superman is not a Nice Guy, he is a Good Guy. He is the classic good guy, and he is a mostly uninteresting character. He is the boy scout of comics. He can't tell a lie. He can't be injured. It's just not that interesting. But he's still nice, right? Well, that's sort of true. I don't think Superman is really trying to be Nice though. Consider his alter ego Clark Kent. Clark is the nice guy. Superman is just Superman, a benevolent alien with immense power working to protect the race of his adoptive parents. The Nice Guy theory actually works the other way with Superman. He designs the Nice Guy persona of Clark Kent to hide his true nature as one of the most powerful beings in the universe.

Captain America is also a nice guy, right? I don't think so. I don't think Captain America's strongest quality is kindness. I think his main qualities were always related to his leadership skills. He is supposed to be an inspiring leader, always at the front of the charge and putting himself in harm's way to protect others. If he comes off as a boy scout or a nice guy, it's mistaken. Though he wields a shield instead of a sword, he still uses that shield violently, and often.

I think writers are generally aware of human psychology when writing their characters. Pop culture tends to be a bit more pulpy, where the drama is mostly related to the plot rather than character-driven, but even pop culture writers use human psychology when determining which characters will be the ones to do certain things to drive the plot forward. The trend I see in pop culture is that the nice guy is always somehow evil. In real life, nice guys tend to be nice to get closer to girls. Either way, the motives for being nice are never genuine. As a general rule, I don't trust them.