So, if a dark-skinned alien with Kryptonian-on-Earth levels of power landed in the Deep South in 1839 and found himself a slave, he'd just break the chains binding him and others, right? Not necessarily. A comment thread on my piece about Icon #16 popped up, explaining why someone never got in the Milestone Media comic-book series after wondering Augustus Freeman didn't use his superhuman abilities to overthrow the system of chattel slavery he grew up under:
I couldn't stop thinking about this question. I don't presume to speak for Dwayne or anybody who was at Milestone past or present but I think this is the classic problem of "why don't superheroes fix everything?"
Icon talking to superpowered members of the Blood Syndicate, who are angry about the cover-up of experimental tear gas that changed them.
Of course, the real-world answer is most often that this would be boring and break the dramatic tension of the fiction being built. Creators have leaned on in-story loopholes to wave off the fix-everything question, like Hitler having the Spear of Destiny and Japanese prime minister Tojo having a piece of the Holy Grail. (There was a latter-day explanation that those leaders used those artifacts to make it so that Superman, Green Lantern, and other ultra-powerful heroes active during WWII would fall under Axis mind control if they operated in territories controlled by the Allies' enemies.)
A while back, I wrote about an issue of All-Star Squadron where the Green Lantern of the 1940s destroyed a telepathically-projected facsimile of Japan. That issue took another tack at dealing with the fix-everything question, positing that the psychological weight of singlehandedly killing millions of people and ending a war would break someone's mind.
As another commenter says, there was likely some element of social conditioning to Augustus' reticence to singlehandedly overthrow slavery. That's hinted at in the panels when Icon says, "I helped who I could. Did as much as I thought was possible." Icon likely wanted just to live as normal and unassuming a life as he could.
Another thing to consider is the fact that, unlike Superman, Icon was an adult lifeform when he came to Earth, a mediator in an interplanetary utopian alliance called The Cooperative.
There were prime-directive type rules in The Cooperative about interfering in the affairs of less-advanced planets. If I recall correctly, he retained full consciousness of his former life. Even if he didn't, maybe there was enough of an inkling of the idea of non-interference in his human mind to hold him back. Why does he fight in World War I & II and later work for justice as a mere human, then? Probably because, as detailed in other issues of the series, that's what felt right.
Future issues of Icon showed parts of the main character's past, from falling in love with a human woman and dealing with their genetic incompatibility to being homesick for his native planet. While testifying about his life on Earth, Icon says he only discovered the depths of his invulnerability after the Civil War had started.
But, all of that is inside the fiction. I think the real world answers to the question are more important, like the fact that heroes who re-order the world/universe in their own image don't often stay feeling like heroes. Even interventions that seem relatively minor and/or altruistic, like ending slavery, would have exponentially bigger ripple effects. If a flying black man starts tearing apart plantations, then slave-owners might start killing other black people indiscriminately for fear of more metahumans in their midst. And the ripples extend ever outward, resulting in a Dakota Universe that doesn't look much like our own. The catalyst for Milestone Media was to create a superhero offering that reflected the reality it was created in, not a radically different alternate universe.
Icon is an alien who was stranded amongst humanity, grew to love it, and later withdrew from it. Both in the fiction and outside of it, he's designed to be a hero who lives with us, not above us. Raquel dreamt up the heroic alter ego to inspire people, not rule over them.
Why didn't Icon just all institutionalised social injustice? Because it's never just one man who accomplishes that kind of change. It always takes all of us.