Looking at the Sv-51, VF-4, and VF-27:
The real answer, I think, is singularity of purpose. Most other VF’s can take on various mission packs for purposes that the original VF design isn’t ideally suited for; indeed, in the case of the VF-25 Messiah there was a desire to make an “all-regime” fighter, good in atmosphere and space, with the ability to take on various Super/Tornado/Armor parts to change roles.
The Slayers all seem to be purpose-designed for limited roles, or one role. The VF-4 was space-optimized, and wasn’t really used much in atmosphere. The Sv-51 was the first operational VF, built in response to the VF-0 series and intended to defeat them. The 51’s Super Parts were intended to assist. The 27 was made in response to plans for the YF-29 Durandal getting leaked to Macross Galaxy, which made them modify their 27 (which was a match for a Super Messiah) to accept its own Super Parts (more thrusters, gas, and missiles) to almost achieve parity with the 29.