It's because he's unpredictable. This is the key to being able to beat your man on the wing. If the defender doesn't know if you're going to go on the outside or the inside then he has to wait for your move and then react, which plays way into Martials advantage because he has lightening quick pace and superb close control. Martial is one of the best going on the outside because he somehow manages to dribble the ball along the touch line and not take it out for a goal kick, it's remarkable.

This is also why when we play right footers on the right wing they seem so bad, because the defenders knows they only have one move and that's to go on the outside so it makes it really hard to beat the man because he can easily anticipate what the attacker is going to do. This is why Lingard/Young rarely beat their man on the right wing and so just usually pass it backwards when they square off the defender