I ran into this issue with my 73 Cuda because I was using 70 bumpers, I cannot give you any real definitive answer other than its the way the two attach the guards. The later models used a different jack which changed the way the bumper was configured as I recall. Also, if I remember right you can make them work but I don't remember exactly what I did.
70 Challenger RT 440-6 EFI, 73 Cuda 416-6 EFI
05 Hemi Durango, 01 Ram 4x4, 14 Ram 2500 4X4, 10 PCP Challenger 6 spd RT, 01 Viper GTS ACR, 52 B3B w/330 Desoto Hemi, 70 Hemi RR (under const)
9 x Challengers. AAR Cuda, 4 RR, 2 GTX, 4 Chargers, etc... (too many to list)