I think everyone is taking my Viking analogy far to serious. My point was simply that if you already know what’s in one direction, why waste time and money on the unknown in the other direction. Going beyond my earth-analogy, space is considerably larger, so the stakes are much more higher than they have ever been to any of us in real life, so again, why bother to go into the great unknown, when the other half of the galaxy is a large web of trade-routes and filled with allies.
Exploring the Unknown Regions would simply be a pointless and unnecessarily risky endeavor when you got all you need elsewhere. Even looking at the map you can sort-of see how the expansion took place from it’s starting point at the core. It builds outwards to meet up with smaller pockets of inhabited planets, and the further out you get to less explored areas there are in between the clusters.
Also, if as joefavs suggested, the Rakatans aren’t exactly friendly, then the Republics ability to expand in that direction is greatly hampered. Like I mentioned earlier, I really doubt that the Republic just flew off into empty space and happened to bump into Kashyyyk, or Nar Shadda, they probably learned of them from someone. It probably started with the nearest planets; Anaxes, Brentaal, Tython, etc. and from whoever lived there they might have learned of other nearby planet. And after thousands of years of this kind of hopping from planet to another based on each planets knowledge of their nearby neighbours, the eventually reached Hutt Space on the far “eastern” side of the galaxy, and so forth.
However, if there never was any planets on the “western” side to start this kind of chain reaction, then that part of the galaxy would remain unexplored. And from what I’ve gathered from the Rakatans wookieepedia article, they’re not to friendly, so they definitively wouldn’t be of any help.
Also keep in mind that this chain reaction of exploring the “eastern” half of the galaxy took over 25 000 years, which kind of makes sense if you consider the scales we’re dealing with here.
However, I will admit that my assumption about them not knowing what to look for is somewhat wrong. But even if they can find the coordinates of several planets in the Unknown Regions I still question the wisdom of just going there for no other reason than exploration. And of course someone clearly have tried their luck and explored, hence there a planets on the map like; Ilum, Csilla, Rakata Prime, Lwhekk, etc. But as evidenced by Rakata Prime, the Republic may have simply deemed it to dangerous to expand in that direction.