I still can't see why Waluigi should be in so badly. His career started as the filling pair for Wario in Mario Tennis, then Nintendo developed the "Luigi's enemy" and the so on "Luigi's princess" with Daisy.
To the date, he hasn't starred any remarkeable games apart from the Party and Sport ones. He hasn't got many fans as you can see reading the top prefered characters in any magazine, webpage, and he didn't popped at the poll in the japanese website.
He lacks the appealing status required for Brawl. If you take a look at the characters shown in the trailer, they are all the top notch of Nintendo and were being requested for years. Waluigi pales in comparison with those characters: Metaknight, Pit, Wario and Snake. He hasn't got the right status to be a Smash Bros character yet.
Perhaps he'll get a place in the Nintendo dream roster for the next Smash, but now, he isn't more than a filler character. The problem is Nintendo's of course, because they don't want Waluigi starring anything (geez, Tingle has a game for himself!) unlike most Mario relatives:
Luigi:
- Luigi was the second character in Mario 2D platformers.
- Appeared in every game of the Mario franchise since its beginning in the Mario Bros arcade except Mario 64 and Sunshine.
- He has a huge fanbase.
- Has starred some games, like Mario's Missing or Luigi's Mansion. Luigi's Mansion is getting representation in every sport game, like Kart and Tennis, so Nintendo wants to emphatize Luigi as a main character. He will, soon or later, go full fledged as Wario did.
Peach:
- Peach is the main reason for every Mario game.
- She has appeared in every Mario game since Mario Bros, from spinoff sport titles to platformers.
- Has a relationship with Mario, as well as Bowser.
- She has a huge fanbase.
- She is the main character of Super Princess Peach, as well as having major roles in games like Mario RPG, Paper Mario, Paper Mario 2 and Super Paper Mario.
Yoshi:
- He started as a "item" in Mario World. Because his cute looks, people started to love him, as he got games far different from the classical Mario ones.
- Yoshi is representing his own games, which are very unique and innovative platfomers.
Wario:
- Wario started as Mario's enemy in Mario Land 2, but in Mario Land 3 he got the main role. Nintendo though that Mario Land games needed a style change, so Wario greediness and powers made these games unique.
- He has appeared in every spinoff game since then.
- Has another franchise, Wario Ware, so he is now in possession of both Land and Ware franchises. Representing his games.
Waluigi is a filler character and he is always intended to make a couple with Wario, but since Wario is going Ware in Brawl, Waluigi is out. He could go representing games, but Party and sport games are already represented by... Mario and his crew (Peach has tennis and golf smashes). And if he will enter solo... so Peach and Luigi could do too, cause they have their own series.
Dr Mario, Pichu, Falco, Roy and Young Link are clone characters done in no time because they haven't time to develop so many movesets while having an excuse for them, so they won't likely appear in Brawl.
Now, in this time and age, Waluigi hasn't anything to say about Nintendo while characters like Pit, Ice Climbers or Game & Watch do. Brawl isn't for putting any character you like in, this game is more like a Dream Fight among the top Nintendo characters... and other ones.