A lack of authority, simply put. I have nothing personally against him, great person, but everybody knows how Malaysia turned out, or Turkey 2010, or Silverstone 2010. He never was able to put his drivers at their place. Imagine this same person negotiating with 12 teams with the sole interest to squeeze as much out of it as they individually can.
You need to have a person who can have a strong hold on the negotiating table. Ross Brawn would be a much better option to be honest.
He is too inexperienced to be honest. Running a team with a sole purpose is one thing, running F1 with more different parties involved then you can fill in a villa, each with their own goals, is quite another.