While I don't know the exact source of that, there is stuff that supports this and I'm sure there are other people who will tell you the same and actually do have sources for it. I'm not frustrated that you have a different opinion, I'm frustrated that you ignore any evidence contradicting your opinion and continue to make claims about Sakurai's criteria that are simply not true, and lies about characters you don't support.
Your first sentence is not an argument, it's not a justification, and it's not evidence. It's just an apology. Saying "I'm sure there's evidence," or "I know there's evidence but I don't know it," doesn't do anything for credibility, it only goes to show your own personal resolve. Even if it was true, that you know 100% there's evidence of that, but can't bring it up, it does nothing for the person you are debating with. You can't just expect them to "take your word for it." There's too many variables that make that an implausible preposition, with the absolute most basic one being "it's a lie."
Your second sentence, I assume is a misguided criticism. I have argued time and time again: they
might be variables. I am not saying they are 100%. If we lack evidence (re-read first paragraph if necessary) that they are or aren't variables (solid evidence. Not subjective interpretations as to why situations came to be,) then the only thing we have to work with are our own subjective systems of valuation in which we rate things based upon what we think, because we our who we are, is important.
The problem is, you're constantly saying things "are absolutely not factors," but you're never, ever supplying evidence. And so for people who look at indeed possible factors, and because of who they are, value them to a noticeable amount, you'll straight out tell them they're wrong as if it's an objective fact. This is on par with saying, "my values are right, yours are wrong, this is fact." You need evidence, (and once again, real evidence, not subjective interpretations of situations.)
For instance, if I say that Mewtwo didn't make it into Brawl because he fulfills the same niche as Lucario, and Lucario was more relevant, I'll tell you that this is my opinion based upon some things I value (fulfillment and variety in niches, relevancy).
If you want to disprove this as a possibility, then you need real evidence.
Real evidence: "Mewtwo and Lucario are completely different characters, and the only reason Mewtwo isn't in Brawl is because didn't have the time to code him. We never chose Lucario over him. We don't really care about how relevant something is either." - Quote from Sakurai, developer, or anyone else of that manner.
Not evidence: "Mewtwo obviously ran out of time for coding, because they're obviously very different. Sakurai just ran out of time." - Noah. (just an example, replace Noah with Obama if need be.)
The second example, the "not evidence," is a subjective interpretation of a situation based upon what you value and what you think is more probable.