I think Keating had Turnbull well-pegged when he said Turnbull was "very intelligent and fearless" - he has complete and utter faith in himself and does not fear the consequences when he feels he's right.
Unfortunately, because of this he also "lacks judgement" - that is he does not consider advice from others and acts independently.

This type of egomaniacal thinking had him involved in the "utegate" controversy and a number of other unfortunate announcements he made without consulting his colleagues.

This makes him a loose cannon and to make him Treasurer would put Abbott in a precarious leadership position.

Turnbull is only biding his time and any cabinet elevation would give him an advantage.

Howard was in a similar position when Reith and Costello were busy eyeing each other off to become the heir apparent. When Reith left, Howard was in a vulnerable position so he stacked the cabinet with Abbott supporters to keep Costello at bay which was another nail in the coffin of the Howard/Costello relationship.

Cabinet selections are seldom made on merit but to meet political agendas.