Zia was a Brigadier, he was not the head of mission in Jordon and nor was he the decision maker but his religiosity is why he was sent there.
He worked on the behest of King of Jordon who had told him that he would make sure that he is not suspended or punished for his actions.
Zia was marked as 'weak' and 'religiously emotional' but his backing from the Arab leaders including King Faisal meant that he would not be kicked off.
Bhutto saw him as a subservient soldier, this is while he was kicking out many of our best and because Zia came across as a follower with similar background (arain), Bhutto choose him as the Chief superseding 7 great worthies.
When Bhutto was arrested, the first thing he asked was, has Zia been arrested too.
You should look into how PA, a truly liberal and secular force was Islamized while our best and greatest were all kicked out. Some of the famous causalities in Gen. Nasir Ahmed, Gen. Air Commodore M.M Alam, Gen. Shah Rafi Alam, Gen. Khurshid Ali Khan.
The ones who were overlooked for the chief role went into depression.