i think all this talk about how effective nukes would be is missing the meat of the issue- if you nuke something, you can't take control of it, ot recapture it later. by the third SW, there was a lot of technology they simply couldn't replace (until the helm incident, at least) and they reached a point where it was too valuable to risk breaking them, even if it mean you lost them.
even in the modern era there's still plenty of "vacant lot" worlds in most of the successor states, and if you try to take a planet it's generally not simply because it happens to have some nice suburbs to colonize. there's valuable resources, and if you bomb them into dust, you can't get them back no matter what. back during the 3rd SW, those valuable resources were literally irreplaceable. i'd hardly call avoiding a weapon that might render the very reason you would want to hold a world moot a gentleman's agreement when doing so might deny the entire galaxy ever getting certain technology again. nobody wants to be the jerk that destroyed the last working 'mech gyro factory because they didn't want to lose it. or the last cheese ball factory, but i don't think that technology was in danger of being lost.