You know, the stream reminded me of something. Abomasnow being added as a Poké Ball Pokémon was one of the more subtly perplexing things about Smash 4.
Generally the Poké Ball Pokémon reflect the most recent generation. Sinnoh was old news, but received five new ones. Palkia and Giratina are easily explained by Sakurai wanting cover legendaries in the game, and Spear Pillar not returning may have led to Palkia being there as consolation, with Giratina being brought along as well since it was left out. Arceus and Darkrai weren't officially revealed when Brawl released, and considering Darkrai's popularity and Arceus' importance, they were solid additions.
...But then there's Abomasnow. He's not really important, nor is he particularly popular (doesn't help that Hail teams are the least common weather team). He's just a late-game Sinnoh Pokémon. I doubt getting a Mega had much to do with it since he doesn't Mega Evolve in Smash. But there he is, bless his heart.
Honestly my only guess is that Sakurai just really likes Abomasnow. Maybe it's his favorite. And hey, it's even pretty dynamic for a Poké Ball Pokémon, having Ice Punch AND Blizzard.