You don't know that, in fact not so long ago you were saying Whithouse was a shoe-in to get the job. He's written eps in every Moffat series after all? Chibnell hasn't.
He's written two episodes this year, was supposed to write three
and "Pond Life" too. He's been allowed to cover the big character stuff that RTD or Moffat would normally do themselves. It's hard not see him as being groomed for the Big Seat.