The primary reason for that is simple… the F-4, F-14, & F/A-18 are all complex aircraft with lots of avionics, etc (as well as different engines from each other and the attack aircraft).

They would have had a hard time fitting the extra support equipment and extra personnel needed to maintain two fighter types and two attack types (most of this period saw carriers with both A-7 Corsair IIs and A-6 Intruders aboard), along with the ASW & AEW aircraft.

Remember… just on engines there were multiple types:

The F-4 had the J79 (as did the RA-5C).

The F-14 had the TF30 (as did the A-7A/B, but these were being phased out of the fleet about the time the F-14 entered widespread service).

The F/A-18 had the F404.

The A-7E had the TF41.

The A-6 had the J52 (as did the EA-6B and the A-4, but the A-4 had been replaced by the A-7).

The RF-8 had J57 (as did the EA-3B).

The S-2 had the R-1820 (as did the E-1 & C-1).

The S-3 had the TF34.

The E-2 had the T-56 (as did the C-2).

The SH-3 had the T-58.

There is a limit to how many different engine types the engine shop on a carrier can support, too many and there is no more room.