Why were proconsular appointments so staggered under the Principate?

Question

Ronald Mellor, in an article about the new Flavian elite (Mellor, Ronald. "The New Aristocracy of Power." In: Flavian Rome: culture, image, text (2003) edited by Boyle and Dominik: 69-91.) rather casually says:

Consuls had to wait at least a decade before a proconsular appointment, and those were anxious years for Vespasian. (p. 72)

Why did they have to wait, actually?

Obviously under the Republic the system was different - a consul who finished his term became proconsul somewhere. Why the change?

Some possible hypotheses:

1. There were many more candidates, now that suffect consuls were in vogue (see here).
2. There were less provinces to go around (since the Senate only governed a few of them).
3. Some kind of probationary term.

(1) might be wrong because I am not sure that suffect consulship really qualified one to be a governor, I think that one had to be a "real" consul to qualify. (2) is indubitable, but was it enough to create a queue of 10 years?

UPDATE: Looks like my hypothesis that suffect consuls could not become governors on the stength of the suffect consulship only was probably wrong (at least, Mellor's detailed prosopography includes several counter-examples to it).

Answer 1

The terms of the Augustan Settlement of 27 BC, where Augustus and the Senate defined respective powers, set a delay of five years from Consulship to receiving a Senatorial Province, so at least half of this gap was by law. The Senatorial provinces were, as a rule, staid and peaceful provinces as well, and had little or no military forces.

The point underlying all of this was to put an end to the Republican mode of using provincial wealth and provincial armies as a lever to contend for ultimate power. An up-and-coming Consul who fancied himself in opposition to Augustus or the Emperor would wait years to see even the most boring province, if any. If you played ball, though, a talented fellow might get appointed by Augustus to a nice command with some legions. So the mode under the Principate was to encourage the leading lights of the Senate to work within the system rather than plot how to overturn it.

An additional wrinkle is that while all Senatorial provinces were ruled by a man named Proconsul, a majority of these officials were of Praetorian rank and served for several years in a row. So the pool for a job after becoming Consul was considerably reduced.