Because, it's not neccesary to the plot, and ash has too many Pokemon to use in every single episode. You have to keep in mind that it's only 30 minutes. He didn't show his unfeazant, for example. The plot can be found here: http://bulbapedia.bulbagarden.net/wiki/BW046
As the angry Beartic shower our heroes with Ice-type attacks
It wouldn't make much sense to use a grass type, now would it?