Upvote:1
The change in number and frequency of the suffect consuls just reflects the changing of the job of consul with the Principate.
Under the Republic, beyond ennobling your family, allowing you to run Rome for a year, and getting the year named for you, consulship was the bridge to a plum job administering a province where you could collect money and contacts that would allow you to pay off the amount you spent getting to the consulship in the first place, and set up for the next generation. If you left the consulship before the job, you would miss that payoff.
When Augustus was in charge, he had no interest in ambitious senators using armies in the provinces to springboard themselves to power...like he had. For a while, he was always consul himself, which caused grumbling because this locked out the aristocrats. The need then was to reward aristocrats for toeing the line, and find candidates for the administrative jobs in the provinces. The multiple suffect consuls allowed a larger pool of candidates for these jobs to be created and allowed more rewards for the nobles to strive for and boost their families status.
So the main reason for the shift was that the nature of the consul's job had changed from Republic to Empire.