Pikachu learned Electro Ball in Unova when fighting a triad of Ducklett. Ash decided that since Volt Tackle was failing him in that season (i.e. Team Rocket, Gym Leaders, and wild Pokemon) he'd instead switch it to Electro Ball.
Because Pikachu forgot Volt Tackle for Electro Ball, that's why.
Maybe its because of the new region...the people that decides on what moves should be shown on tv/game wanted to introduce the new moves like discharge and electroball.
