I think it's because their entire system of philosophy is in a constant state of collapse. Corporations bend over backwards to cater to their worldview, but they always need to be the radical resisting the system in order to maintain their self-esteem. Even if complete victory were handed to them on a silver platter, they'd move further into the esoteric left because they'd lose their clout if they stop being an activist. Their whole ego is built around deconstruction of the system. If they run out of things to fight for, they need to make new causes up.
I also think this is a major factor behind why communism mysteriously never works whenever it's actually implemented.
That's part of the reason why it's called "progressivism." They don't have any end goal in sight, it's just "progress" for the sake of progress, whatever it means. That's why western leftists will always gravitate towards societal entropy, because they've got nothing else to do.
It's also that they are so caught up in their quest in trying to find a fascist bogyman that they have to twist their brains in order to create them out of imagination even if it doesn't make any sense at all or is counter productive.
As I read in the Salon article, among their arguments is that superhero fiction reinforces the status quo rather than challenging it, and even compares it to Charles Dicken's take on capitalism, which is that capitalists should be kind and generous, not that workers ought to be rebellious.
Strong men who willingly allow themselves to be fettered by their strong sense of morality and justice, rather than little men trying to tear them down for the sake of "fairness." An authority figure or business owner being kind, fair, and generous is unthinkable to a leftist, they're all evil and bad.
Leftism teaches that the little people should tear down powerful or gifted people merely for being powerful or gifted. Not that powerful or gifted people should lift others up.
At the risk of sounding like a bit of a sped, I think the Incredibles did a really good job at illustrating why exactly leftists might not like superhero fiction:
You have the antagonist, Buddy/Syndrome, who is angry and jealous at superheroes because they have what he doesn't have. Superpowers. He mistakenly thinks that it's the superpowers that make them special, when in reality, it's the fact that they choose to do good with those gifts, rather than self-aggrandizement. His ultimate goal is to sell technology that would level the playing field for normals so that "everyone can be super" so that no one is. To tear down those not only more physically stronger and more powerful than him, but also those more righteous than himself.
