I don't know what happened with George. In the series, he was believable and did what few character scenes he got really well. But during the films, his style changed. Maybe it was all the convention appearances. He developed a pompous, orotund speaking style that younger Sulu never showed. He started sounding increasingly phony, as if in love with the sound of his own voice.
That's it; I'd say you put your finger on it. Having been a bloviator in local politics and an eagerly applauded speaker at ST conventions, Takei infused film-Sulu with a level of vanity and pomposity that even Shatner himself couldn't carry off. That's why film-Sulu seemed mannered and artificial.