Most of the planets we see in the OT are either on the fringes of the galaxy or have been picked by their current occupants because they are out of the way.
The planets we see in the PT are mostly a part of the Republic, which is still booming at this point, so I can see why they were already chock-full of people. I always thought the tech issue was that, when martial law is imposed, the money just went into the war effort, hence the impoverished look of the galaxy - apart from Cloud City, which is a pimp's paradise.