Two simple reasons I can think of
1) Due to the Nature of the MMORPG, players are less likely to make a choice that involves them losing. Whether they are losing money, xp, or just the sense of loss. Dark side bribery rewards are so minute only the minmaxiest of minmaxers will take the money. Losing money, no matter how minute, is a little more feel bad, and so was likely not implemented
OR
2) Say you just bought a huge upgrade or three for you gear, you are blazing along having fun, get into a conversation, try to take the light side option and.....you can't cuz you ain't go the monies. HIGHLY UNLIKELY, but more like a fail safe than anything.