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.
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.