#1 He doesn't have the full confidence of his manager
When Jose Mourinho arrived at Manchester United in the summer of 2016, Anthony Martial was one of a few shining lights for the Red Devils, and he was seen as one of the players who would lead the club into the future.
However, two and a half seasons on, Martial has failed to establish himself firmly in the squad, as Mourinho has repeatedly shown not to fully trust in the abilities of the Frenchman.
Mistakes are bound to happen in a player's career, even to the greatest of all, and it is a supportive manager's duty to make his young players understand this fact, and aid them in moving on from the setback, as players at a young age haven't developed the necessary emotional maturity to deflect negative opinions from an unforgiving media and football fans. If not properly handled, the player might never recover from an early career setback, and fail to fulfill his potential.
What he found in Mourinho, however, was a manager who took out time to highlight all of his flaws in public, questioning his work ethic and commitment to the United cause when Martial took time off to spend time with his family after the birth of his second child.
Martial has seen his playing time reduce drastically, as Mourinho prefers to field other players, and for as long as the Portuguese remains at the helm of the club, it would be an unwise decision for Martial to continue playing there.