Section 814 of the FY2007 National Defense Authorization Act (NDAA), Pub. L. 109-364, required that, in DoD, award fees be linked "to acquisition outcomes (which shall be defined in terms of program cost, schedule, and performance." It also required a number of additional standards and controls on the use and administration of award fee contracts.
Section 867 of the FY2009 NDAA, Pub. L. 110-417, effectively extended most of the Section 814 requirements to civilian agencies.
It's possible that if you've noticed a trend away from CPAF to CPFF, it may be because folks haven't fully figured out how to comply with these "new" requirements yet. Alternatively, it could be viewed that CPFF is just "easier," particularly in light of the fact that AF contracts have been coming under greater scrutiny, thus demanding more time and attention to get things right.
Personally, I doubt it's due to "administration costs," but I have no data or specific information to either prove or disprove that.