I think there are a number of reasons and they may differ dependiing on where you are talking about.
e.g there is no shortage of classic pilots to work in Oz, there is a shortage of Captains in the EU.
The pay is not great (in comparison to FO starter salaries in all areas), US based guys are leaving (744) and the command course seems not to favour freighter promotions (US).
Also it is a time of expansion so there is greater demand for crew.
I'm not sure Rapid commmand has actually happened.
With a number of bases and types you see how it is difficult to answer such a general question.