Because it's ****ING BORING. Doing the same **** over an over again, and never taking the initiative to try something different is what makes games go stale. Example, Call of Duty.
There would be no point in adding a character like Deoxys or any other similar transforming character if it's just gonna be a variation of the Pokemon Trainer Strategy.
My advice, go play some other fighters, especially the Arksys ones like BlazBlue and especially, Guily Gear, and pay attention to how much thought is given to each character's uniqueness when it comes to playstyle and combat strategy.
When you introduce a NEW character, they have to bring something NEW to the table on all frontiers, not just appearance, size/weight/speed/power and movepool, but also in fighting style and overall strategy. This is the main problem with Lucas (an a lot of Smash Characters), he *feels* very similar to Ness, sure, his moves and properties are different, but over-all he's like a mid-long range "specialist" Ness. Every time I played Lucas, I'd just get the urge to play Ness instead, as Ness felt more complete. This is the same exact thing that's gonna happen if you introduce another character with the same mechanics as an other, but with different moves; the over-all strategy of the character is too similar, and thus the character suffers for it, making the character very "disposable." Wolf to Fox is an example of a transition done well. Both characters seem to be quite similar, because their moves have a couple similarities here and there, however, their play-styles and strategies are COMPLETELY different. Same for Lucario.
In short, there are many other ways to introduce the transforming gimmick, just like there are many other ways to introduce the "duo" gimmick. No need to just copy/paste what we already have. The taunt transforms, and Specials transform are great ways to go about it and bring up something new and fresh.[/qoute]
Personally I don't think there's any need for another transforming character. Unless toon zelda or another pkmn trainer are added. And as boring as it may be consistncy works wonders, especially for a causual fighter like smash. Is it a cop out, sure. Is it lazy, definitely. But in the end that's better than over complicating a character that really doesn't need to be made so complicated. I remember that there was a similar discussion on masked link and this kid put an idea that each special attack changes him. It wasn't bad but it really make the whole character a pain to use, and even make. And honestly that's not true to that character anyway so it was dumb even in that regard. Though that could actually work in deoxys' case but I digress. I don't wanna see deoxys in smash anyway but if it happened and it was the simple down special transforming thing it really wouldnt be as bad as u seem to think it is cause its a proven successful formula