[Theory] Why is Red silent?

Seriously, he's all laughing and jumping in Pokemon Origins and then in the games, speechless.

Imo at the time it was simply because he was the player character in the first games and they didn't want to "give him" a fixed personality as the "secret final boss" in gen II so everyone could imagine their own dialogue/their own character if they wanted.
Or simply to give him a more misterious/cool vibe as the "strongest trainer".
After that it's become pretty much iconic and suddenly hear him say anything or being all chatty would be weird.
He's just one of those "strong and silent" types.

But then why did they make him talk in Pokemon Origins in the first place?
Judging by your answer, we aren't in a place to call him 'Red' at all.

I don't think the Origins one is supposed to be the "same" Red we encounter in the games, they are different versions of the character.

Different media forms have their own take on characters. I'd say they decided to give him a voice over time, however sun and moon reinforced Red being silent. So really it just comes down to different "universes". Red in the manga is different from both the games and Origins, he's more chatty than in the games but more serious than in Origins.

All the Pokemon protagonists are silent, Red's just the only one who shows up in other games (afaik)
and yea different media different canons

Easy, after a while he began ordering his mons using his mind, and then he did it so often he forgot how to talk.

Or maybe he just prefers not to talk.