I think the reason the crystals chose Sunny specifically was because she was being more of a leader than anypony else was being, and as such she was the pony most worthy of becoming the next Princess of Equestria. Think of it this way: for decades (if not centuries) all ponies decided to not only stay away from each other but also spread fear-mongering propaganda about each other. Nopony ever made any attempts to restore friendship through the reunification of the tribes. Sunny was the first pony to actually believe that restoring friendship for all
species was possible. And since her friendship with the other Mane Five ponies is strong, her magic is likewise also
strong. We saw a bit of this in the ‘Make your Mark’ special, as well. It will also likely be the thing that enables her to win against seemingly impossible odds in the future, as well (won’t say anything beyond that, as I don’t want to spoil anything).
Also, that is a fantastic drawing of filly Sunny. She looks truly adorable there, Pinto!