It's because he is physically soft and slow. Dribbling gives you an advantage, but if you are not at least reasonably quick, you are going to lose that advantage very quickly. I remember Felix almost escaping 1 vs 1 vs Madrid at 1-0, but rudiger caught him from behind so easily, it seemed like an uneven contest of speed and strenght.
Same problem as Pedri to an extent. Slow players rarely reap the fruit of their technical ability. Makes me think Iniesta was not that slow and weak as some people think. Either that or he really ranked 10/10 at technique and intelligence.