Its Mourinho's peculiar form of man-management. It is part revenge, part a desire to see Martial 'man up', endure pain and come through it and prove himself a warrior dedicated to the cause. He will want to make Martial run through walls for him and really want to start under him.. he'll mess with his head to test how loyal Martial is to him and whether he can handle being dropped and still listen to him.
It is a battle of egos and in all fairness to Martial he has handled this all with aplomb this season thus far and doesn't seem to be running to the media forcing Jose's hand or overcelebrating, being selfish in games.. trying to belittle the manager or his rivals for the position. He's acting like a model pro so far and we just have to hope he continues doing this and the performances keep staying at this level, as once he earns Jose's trust, Jose will die for him and start trying to convince Deschamps to play him for the NT.
I genuinely think Jose can grow to love him and that it is more an emotional reason. man-management/psychological reason he keeps dropping Martial rather than it based on performances or even tactical. He wants Martial to be his soldier and a son for him, then he will unleash him. For now Martial is still in his bad books and needs to keep fighting to get in his good books. See the case of Mkhi last year and see the case of Shaw now - a total write off in Jose's books IMO.
Jose is very touchy about stuff like this.. to most people it can seem batshit crazy and irrational, but it is his way of management. When he feels like he doesn't have that loyalty from his players, he goes nuts.. and loses the dressing room.