I don't even think it looks cheaper. The Tatooine scenes have an incredible crispness to them, and it looks lived-in in a way the latter two movies don't. It's just... different.

It's kind of like how movies from different eras have a different look to them. I can discern if something was made in the '80s or '90s just by looking at them--with some exceptions; Se7en looks like it was made today. Jedi definitely has the look of an '80s blockbuster, Empire is very much a mid-to-late '70s film. It's just the first that looks out of time; not to its detriment, mind you.