Summarized: The Republic never got very far into unexplored space because charting new hyperspace routes is very time-consuming and dangerous. Aside from that, they're facing a galactic crisis practically every few decades. What kind of Republic would have the time to explore under such circumstances?
"Grant Allen [...] had written a book about the Evolution of the Idea of God. [...] it would be much more interesting if God wrote a book about the evolution of the idea of Grant Allen." ~ G. K. Chesterton, The Everlasting Man