They just wanted to show the gangs pokemon all together for the foreseeable future. If he had more then 6 then his pokeball would be locked rendering him incapable of using them. Like they showed in BW018 when Seawaddle was caught.He caught the Krokorok, besides, isn't he supposed to have 6 pokemon??? Plus this is a soothing song, I haven't watched Pokemon in a while, but it's good that Tepig, Dwebble, Roggenrola, Tranquill, and Swadloon evolved(in the upcoming Japanese episodes).