The government of the Roman republic relied on collegiality; there were, apart from the dictator and religious offices, no single decision makers. Each magistrate had to have the consent and of his colleagues and of the senate before making a decision of any kind. Thus, the senatorial class could always control ambitious politicians and prevent a concentration of power in the hands of a single person.
However, this system wasn't applied after the first conquests outside of Italy. Initially, the provinces were administrated by praetors; then by proconsuls; then by legati augusti, prefects, procurators and proconsuls; finally by praetorian prefects, vicarii, proconsuls, consulares, correctores and praesides (civil administration), magistri militiae, comites and duces (military command).
Each of these officials administered his office with the help of a staff (sometimes bigger, sometimes smaller), but was never controlled by a colleague like magistrates in the city of Rome were. Magistrates governing in Rome proper were watched over by his colleagues, by the people's tribunes and by the senate. The provincial governors however had nearly absolute power in their administrative circumscription and could plan and do anything, ranging from an illegal war over extertion to usurpation and civil war.
It's true that they could be charged after the end of their term of office, but often such accusations lead to nowhere since the witnesses were dead and the judges bribed by the defendant. But if the office of provincial governors had been collegial, it would have been much harder for a proconsul to do any major damage.
So what if the Romans applied their principle of collegiality also to the provincial governors?