The SW galaxy has hit a technological plateau, where they can't advance beyond FTL travel and laser guns(I sure can't think of anything more advanced than lasers). That armor look for Republic Troopers is probably intentional, since it's a successful method. Our armies have been wearing the same basic battle fatigues for 50 years, appearance-wise. There might be some different tech under the skin(like kevlar), but on the outside it looks like the same environment-appropriate camouflage cloth. Maybe instead of asking why the armor is the same 3500 years in the past, you should ask what's so great about this armor that it's still being used 3500 years in the future.