That was because we lacked knowledge about the biology and couldn't see opportunities for interventions. Part of gaining new knowledge is better understanding of how to overcome problems, but the other part is about learning limitations. Our current understanding of cell biology is pretty decent, at least good enough to say that it'd be incredibly difficult for a therapy to repair large amounts of radiation damage.
Something cool that could work though: New strategies for treating stroke involve putting genes that protect against hypoxia into the brains of people by way of viral infection. Think of it as a vaccine against stroke, if you will. This way, when you have a stroke, these exogenous genes activate (from hypoxic stimuli) and form proteins that will mitigate damage from ischemic stroke. If you did something similar for radiation, you could up the baseline tolerance of people to radiation exposure. Sure, it won't do nearly enough to save you from the massive exposures that would trash all your DNA and the newly introduced anti-radiation genes, but it'll help with mild-moderate exposures.