Maybe because of his poor record with talented youth in his previous clubs.
The fact he's singling out Martial and Shaw for public criticism, yet he's ignoring poor performances from other players.
Because it seems that same standards don't apply for everyone. Martial need to be "magnificent" in order to start, whilst some other players are allowed to be very poor.
Everytime Martial starts playing good, he gets dropped for the next game.
By Mourinho's own words he said he must play Mikhi after his performance vs. Wigan (agreed, Mikhi is class) but when Martial was MOTM vs. Stoke, Middlesbrough, West Ham, he got dropped the next game.
I don't buy that "motivating, challenging him" crap that's being served because even to the most die-hard Mourinho supporters, his behaviour is quite baffling.