I'm not a big PT apologist, though I enjoy it more than most here, but I think the answer is in the OP. As is the case with the way technology appears and the way the whole galaxy appears to be a busy place (vs. the OT where there are more lonely planets and technology actually appears inferior), Tatooine seems to have succumbed to decay. Before it still wasn't a booming planet, but it was certainly a center for some activity. By the time the OT rolls around, it's like a planet-wide ghost city.

Note: I am not a fan of this explanation. In spite of the evil of the Empire, I have a hard time believing there'd be such galaxy-wide technological decline, especially considering that it's not consistent (i.e. bigger ships, Death Stars, etc.). I can see the lower standard of living, as such is the case in present-day dictatorships, but military technology clearly should be improving, as that is often why dictatorships have such poor people--their military might is disproportionate.