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?
"There is something going on in time and space, and beyond time and space, which, whether we like it or not, spells duty." -Winston Churchill
"For of all sad words of tongue or pen, The saddest are these: 'It might have been!'" -John Whittier
"Apathy is death." -Kreia