Why only 6 AAM's for the F16?

I have been watching the outstanding heavy weapons load thread and time and again pictures come up of Super Hornets carrying upto 12 AAM’s. It got me thinking as to why the F-16, and the F15 to an extent only carry six and 8 respectively. Surely these aircraft could be fitted with the same twin rail launchers used by the F/A-18???
With the Falnker and even Fulcrum now offering the ability to carry large numbers of AAM’s surely it would make marketing sense to up the load on the Eagle and Viper?