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:
- There were many more candidates, now that suffect consuls were in vogue (see here).
- There were less provinces to go around (since the Senate only governed a few of them).
- 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).