Was that uprising such a defining event to dominate the region's politics relative to the rest of the nation for so long?
It is more the harshness of the crackdown made it clear to people in the SW how the rest of Korea looked down upon them. And since the establishment was Center-Right it made sense for SW to consolidate around the Center-Left. Also leader of the Center-Left Kim Dae-jung also hails from Jeolla. In elections of 1980s and 1990s the Center-Right also ran on an anti-Jeolla platform by saying that ROK cannot allow someone hailing from the "backword" Jeolla like Kim Dae-jung become Prez. All this served to consolidate regional and ideological position in Jeolla toward a Left direction.