One reason to delay the dynamic reforms and have a minimum year requiremnt was to have some time between conquets and the availability of legionaries outside of italy, but after the specific reforms happened that is of ourse not longer the case.

The outside territories far away from Rome and Italy were excluded from legionary recruitment becasue romans will probably only conquer them very late in the campaign, thus don't have time to properly romanize them, especially since these provinces are often steppe or desert and simply very far away from the heartland of roman civilazition. And with such an big empire they will probably have enough resources for more manpower near the theatre of war at this time.