I agree that it's strange; but it makes sense as Pikachu is one of the most popular Pokémon in the series. You might as well start off with him, especially from popularity of the original animated Pokémon series.
It would also set up the universe in a sense..but who knows at this point. All I'll bet (besides not wanting Danny DeVito to voice Pikachu), is that Universal will probably help distribute the film.