I think a lot of it has to do with the type of combat that the designers want to encourage. In the Star Wars movies, there are many battle scenes that feature combatants hiding behind cover, popping out and exchanging fire. Giving a big bonus for cover - especially when most players won't get much from armor - is a good way to encourage this kind of cinematic fire fight.
D&D combat is not like SW combat. In D&D, they want lots of guys in heavy armor with big swords, battling it out for glory and honor. If you give large bonuses for cover, you're giving players more incentive to focus on ranged attacks. Ranged combatants already have an inherent advantage, and 4e gives them another one by increase overall mobility in combat. So maybe they think a +5 cover bonus on top of that would make ranged combat a little too enticing.
D&D combat is not like SW combat. In D&D, they want lots of guys in heavy armor with big swords, battling it out for glory and honor. If you give large bonuses for cover, you're giving players more incentive to focus on ranged attacks. Ranged combatants already have an inherent advantage, and 4e gives them another one by increase overall mobility in combat. So maybe they think a +5 cover bonus on top of that would make ranged combat a little too enticing.