Let's consider the settings for the two trilogies.
Episode I-III
- Naboo, a prosperous and peaceful planet
- Couriscant, galactic center of commerce and industry
- Tattooine, a schizo-desert planet with dangerous pod races we never see in the original trilogy.
- Some red planet I can't remember the name of that has a huge droid factory on it.
- Another far-away planet that is mostly storm inhabited by strange aliens with an unusual obsession with cloning and no background checks on their clients.
Episode IV-IV
- Tattooine, a desert planet without pod races, but with a fairly large spaceport named Mos Eisly that has a droid-detecting device built into bars to keep them out, and Jawa crawlers dealing in scrap droids like R2D2 and C3P0
- Yavin IV, a moon that is mostly a jungle
- Hoth, an ice planet, completely inhospitable
- Dagobah, a huge forest planet, plus swamp
- Cloud City, a city that literally exists in the clouds harvesting precious gases and other materials from the planet below, with a fairly advanced security system
- Endor, another damn forest planet
- Two Death Stars, massive space stations the size of small moons
A lot of the major locations for the second trilogy don't have much reason to host high-tech gagetry, so it makes sense that we don't see a whole lot of it. It also makes sense because these movies were made before the I-III trilogy, with a lot less CGI, so they weren't capable of showing as much high-tech wizardry.
That being said, when we DO see examples of 'modern technology' in the IV-VI trilogy, it is massive and amazing - entire cities floating in clouds, space stations that have been decades in the making that are the size of entire planets. Not to mention the incremental improvements mentioned in DarkHeart's answer.
It also helps to keep in mind that the original trilogy is largely focused on the rebel alliance, which is far-flung and stationed on remote planets not likely to have access to high-technology. The empire is much better equipped, but we don't see them as much.