there are many reasons to this. im glad that you didn't just shout at something like: MEWTWO IS NOT IN BRAWL ALL YOU PEOPLE ARE STUPID HIS ICON DID NOT APPEAR! cus those things make me mad
1. The trailer, or at least that part of the How to Play Video, could have been made a few months ago. A few months would be enough time for bug testing, AND MAKING Mewtwo. So, may be they did not put him in the game yet.
2. Mewtwo cannot be powered up by groudon. May be it's because he's an evil character, and you cant play as him, cus he is like Wario/Bowser with the Trophy Guns, so he does not appear.
3. If Mewtwo is playable in SSE, it could be the type of Pokemon he is that Groudon does not power him. May be Groudon's effect was something like: Less knockback, which makes sense cus Groudon is a Ground(or mud?) pokemon, and he is big and heavy (translators, could you translate the description of Groudon's sticker please?), and the knockback may not apply to Mewtwo because Mewtwo hovers insted of walks or something like that. May be the sticker powers up you Head or something, and Mewtwo does not fight with Headbutts, etc.
4. They could have been blanking out the icons, but they only blanked out Mewtwo (and may be more?), but forgot to blank out the rest. Or may be the people who were responsible for making that part of the video were trusted, but they did not finish in time or they forgot to finish it, and there was a lack of communication so the people who presented the video trusted that they'd take out anything that would leak info, but they didnt know cus they were not contacted in time. So that is another reason.
5. This is not true and those who posted the video/pics were just tricking us (probably not likely)
6. This is not true because Nintendo did this on purpose just to entertain us so we're more patient, or it was a test just to see that in case IF THEY DID REALLY leak info by accident, they would then know how much time they would have to fix it. Or they simply wanted to trick us for fun.
7. They might not have unlocked Mewtwo yet when that part of the video was taken place.
8. Mewtwo's just not in SSE, he's a secret secret character. 1st 'secret' because there are secret branches off of the SSE where you unlock some secret chars. 2nd 'secret' because he's not even in a secret branch in the storyline.
If Mewtwo's in Brawl, i think the biggest reason he was not with Jiggly, Pika, PT, and Lucario(may be hes not Lucario tho!), is because he's an evil char in the SSE and therefore you can't play as him.
If there is a special mode you can unlock where you play on the evil side of the SSE instead of the good side (Mario, Kirby, Yoshi, etc.) then a reason why he is not able to be powered up by GRoudon is just cus you can't power up evil chars in the evil mode.
What makes me think that its something related with Nintendo tricking us/testing how fast leaks get around, is that the DOJO said something like: 'Ness, who has been in the Smash bros. series until now'.
But then again, that statement could also mean 'Ness, who has (known to have) been in the Smash bros. series until now, (BECAUSE HE"S A SECRET CHAR OMG!!!!, and you guys don't know about it yet so that is why i said 'until now' "
), at least. Ya did see Mewtwo Strikes Back, right?
