Zambellas wasn't going to make CDS for two reasons. First, his CV lacks the depth and "purpleness" the post demands; all but two of his posts were Navy centric and, as Bismark said, he has always been too tightly focussed on the FAA to be seen as a balanced Defence Cheif.
More importantly, his ill-conceived and inappropriate foray into politics effectively meant that he ruled himself out of the running. Whilst his articles in the Telegraph were excellent, stepping over the line and trying to dabble in the Scottish Independence Referendum achieved nothing better than to embarrass and piss-off the Government, especially his own minister. Fallon was never going to recommend him after that. The episode was made worse by the fact that his statements were then strenuously denied and held as incorrect. The Government was never going to appoint a man that attempted to speak for them - out of turn.
He has been a good First Sea Lord and I have always held him in high regard, just not what the Government needs for CDS - and perhaps the other two Services too.