Well, the EU explain that most of Clone Wars era tech was relegated to backward planets. The whole Tarkin doctrine of fear inaugurated an era of impractical giant weaponry, like the DS, giant walkers, etc, etc.
Also Palpatine was a miser, maybe due fear of uprisings, speculating here. That's why the bulk of its troops were given cheap gear and equipment. But then again we had expensive shit like gunboats, tie phantoms, darktroopers, tie defenders…