Well - English dub would add around €20 to €30 to the game (voice actors are expensive, and certainly if you need a lot of them). I think the game is at a acceptable price point now, and I personally would not like to pay an considerable amount of money more for this game just to get English voice acting. I think the publisher (and developer) also have realized that the sales would not be great after adding expensive voice acting to a game that's already years old.
I personally am used to Japanese voice acting, because I play, and have played, a lot of JRPG's and VN's. So I don't mind it that much (if at all). At the end it's just a question of taste if you turn voices off or not. I myself have added Japanese voice acting to the Trails in the Sky series also. I prefer that to that "beepbeepbeepbeepbeep" that's in the voiceless version.