Did they actually just let iJustine on stage? Threw up a little. She shouldn't be let anywhere near anything to do with gaming since her disgusting excuse for a 3DS review.
Really? Hearthstone over Smash Bros. 3DS and Bravely Default?
Hearthstone? Really?
Hearthstone.
Have you played Hearthstone? Hearthstone is basically Magic but simplified, and considering Blizzard's Fanbase, the Easy to understand/hard to master gameplay, the fact that EVERYTHING can be earned through Gameplay unlike most other Pay-2-Win Bullcrap, etc. Plus, The other two are, in no short order a inferior in the eyes of the Gaming Press version of a Wii U game and a JRPG. Both aren't as loved as something Blizzard Produces.
TL;DR: Hearthstone won because it's a Fantastic Game AND it's made by Blizzard.