Tua's turnovers are mainly the result of his own limitations, which opponents exploit. Josh does not have those limitations, so his turnovers are the result of him not playing to his full potential (whether that's his fault or his coaches' fault) and not something the opponent is doing to him.
Hence, Tua is fulfilling more of his potential than Josh is, and so he "looks" like he's playing better than Josh, even though Josh at 80% efficiency is still better than Tua at 100% efficiency. And that's why he's getting more MVP talk. Well, that and the fact his team has more wins.