a lot of the quality difference comes from the fact that the VAs are just paid more in japan, and the animation is fitted to their voice tracks. since the american market mentality for anime is mainly to get the popular stuff out quickly, less time/money is put into making a good dub.
also, most of you don't know japanese so you don't know that some of the original VA's actually sound bad/don't fit the character.
another part of the reason we're used to thinking the original is better is because only the better shows get rawed/subbed, while the majority of the shit anime are unknown.