Well, because it's real. A radical story based on a true story has an authenticity that a fictional one might lack. For example, if it's about a real social injustice like the civil rights movement. People know it actually happened, so it hits harder.
A radical true - story - based work can be more impactful as it often has real - life consequences and ramifications that are still relevant today. Take a story about a real - life environmental disaster. The fact that it's true makes the message about environmental protection more urgent. We can see the real damage, the real people affected. It's not just made - up scenarios in a fictional story. This makes it more likely to inspire real - world change.