May be Pikachus are rare and very territorial. In that way it would be hard to find. May be they dont get tamed easily. like Ash had trouble with Pikachu in the beginning, whereas all the starter normally get tamed easily. So it would not be a good one to choose to train.
Then again I also have not seen many chikorita, cindaquill, totodile, treecko, mudkip, torchic (or any of the starters) much in the anime!