Military technology Joneses
Military technology tends to stay static until it meets opposing technology head on that it can't defeat.
The aircraft of world war two didn't change until they met up with MIGs. Since then there has continued to be very little change, the continuing next generation development notwithstanding.
The rebels/republic didn't and possibly still don't have what it takes to build weaponry superior to what the empire already has, so there's no reason to build fighters etc that are a radically different design, or which incorporate design elements from the rebels.
Additionally, it's clear that in the Star Wars universe, they never invented the transistor.