OR... It is just a commercial. LOL...
BUT, I think Snake's world is threteaned just like everyone elses, so he is forced to fight. Like everyone else. the Primid are just running ramped throughout the universe (galaxy, wtf ever), engulfing chunks of worlds. When they threaten a characters world, previous characters chasing the primid catch up, and wtf ever. I actually like this idea...
(making this up completely)
Samus Aran has BEEN chasing the primid for a little while now.
Being a well-known bounty hunter, she has connections with other organizations (I.E. Starfox & possibly whatever Snake works for). So that takes care of them.
Mario, Kirby are 'dueling', and Peach and Zelda are there just to watch. Meta Knight knows about the smash bros. organization** and leads the Minister and his armies to the stadium, which will be his first target. OR, perhaps, Meta Knight agrees to help the Minister if he helps relinquish Kirby. Which would explain why they blasted Mario out of the stadium, leavng Kirby to fight Petey. And they were using Zelda and Peach as leverage to keep Kirby from just running away (like he would). OR, Bowser also has an agreement with the Minister, in that if he kidnaps Peach for him, he'll do ect., as well as with Ganondorf.
Palutena see's this coming, and sends Pit to stop it from happening.
The Triforce let's Link know of Zelda's danger, and he sets off to help as well. Him and Yoshi are both in desert, where Mario might land, as said above...
Ness's world is threatened, so he sets off to save it.
The pokemon trainer is attempting to catch Pikachu (lol, just making up stuff), when the Primid attack Kanto, successfully detonating the bomb. With not much left, the trainer sets out to discover what happened, as the Pikachu gets kidnapped by the primid along with the blast (maybe?!).
DK's island is threatened, so he must also chase after the primid.
Samus or Star Fox contacts Cp. Falcon and informs him they also need his bounty hounter help.
Meanwhile, Bowser, Meta Knight, and Ganon (and any others) are betrayed by the Minister, and cast out of the Halberd. Before doing so, one of them manages to set free the loyalties...
I think I missed some characters, but...
Hey, I just made this up, so don't be TOO harsh...
But, please tell me what you think.
The smash organization**-Master Hand set up a tournament to see who is the greatest (?? lol).