In truth, yeah it partly has to do with specialties -- some are in really short supply and more likely to get recalled & deployed. (I think military police and helicopter mechanics are in the shortest supply, but there could be other tight specialties as well -- intelligence, medical, even truck drivers, etc. And Special Forces fall into their own, unique category.)
In other cases, there are people (like me) very willing to serve, but some of the administrative people are really slow at putting through our paperwork and getting us the schools we have to have in order to deploy.
So basically, there's a whole stew of complex reasons why things work like they do, and not all the reasons are good ones, I'm afraid.