Pokémon games have traditionally relied upon sprites for the character and monsters themselves. There is a reason why the game’s haven’t fully embraced 3D in its graphics, at least not yet.
“For the Pokémon characters, one of the reasons we use sprites, at least currently, is because we like to have more of a comical kind of look, like an animation. This kind of visual style is what we want to do for the Pokémon games,” said Pokémon Producer Junichi Masuda. “If we were able to take that style we have now and have it translate into 3D with no problems; that is definitely something we would be into.”
“It’s not just about the visual appearance either,” he went on. “The feeling and the control of the character is also very important to us. There is definitely a certain feeling you have when you control these 2D characters. It may be different if we switched over to 3D.”
Source: Game Informer