I think the reasoning for that is obvious.
The reason why people struggle to see a far simpler and more logical explanation for Martial not playing is because his ability, and now, performances, do nothing to support such a decision. There is no confusion as to why Lingard doesn't start. There would be if Pogba or De Gea was not playing.
Martial will start many games this year. After all, 'it's a squad game', and we 'have many competitions'. The curiousity is why such a talent is not a key member of our first XI. A simple footballing argument is difficult to make, based on what we have seen not only of Martial since he joined us, but of all of the players who play ahead of him too.
Do you think people have just drawn Martial's name out of a hat to ask questions when not selected? Do you think we could have just as easily asked the same questions about Romero? It isn't complicated at all. Martial, in his first season, was our best forward player. In his second, he certainly wasn't, but one, he had already set a great level with us that we know he can reach, and secondly - NONE of those playing ahead of him managed to make a much more compelling case either. Those who are playing ahead of Martial have hardly been consistently brilliant themselves with us.
Perhaps Martial CAN play his way into the team at some point this season, who knows, but if he does - he will have to do so performing at a level far above, and for far longer, than anything his competitors had to do to earn their own spots in the first place. You would also imagine that to keep said spot, he would also have to perform far better than his competitors have been able to perform in order to still keep him out at the moment.
This then brings us to the theory of him leaving. You would imagine that abplayer of his ability and promise will not be content to feature so sporadically, 20mins a week if he is lucky. If it continues, while performing as he is, he will likely feel he deserves more, and if he isn't getting it here, he'll go elsewhere. I imagine Jose will then come out with his speech about it all being Martial's decision, and how he won't stand in his way if he wants to leave. If he's not getting a look in, of course he will want to.