If I had to take a guess, I think they chose to use subtitles because of scene direction. A lot of up close shots and some POV shots too. Maybe they felt that text bubbles would obstruct those type of scenes on the tiny 3DS screen? I don't think the scenes look that bad. The trailer does seem more story focused than LoL but I'm not expecting much from the story. I'm much more interested in gameplay changes as the minimal story in LoL did not bother me.