The difference probably is that the Chieftain and Challenger are actually useful ships. They can be turned into potent fighters. For sure not on par with the FDL, but still powerful enough that you can use them without feeling like just fooling around.
This means that when those ships came around, enough people used them to justify the implementation of ship kits. In contrast, the Crusader really never got much use. When not using the SLF, the ship sacrifices a lot (internals, shield, speed, agility) for a marginal increase in hull and firepower. When using the SLF, it sacrifices even more of its very scarce internal space. And if you want to be able to use two SLFs at the same time, you give up a lot of shields.
All in all, the FGS already would be the superior choice. But the Krait MK II also was introduced aorund that time. Sure it costs a bit more, but it also is vastly superior on any aspect. Which means that only people who really, really (and i mean really, really, really!) unconditionally love the Crusader for other reason than its performance use this ship.
Its reception was so bad, so few people fly it, it just makes no sense to invest time and effort into making ship kits for it. They would probably not sell enough to justify the investment. So what the Crusader actually would need first would be some buffs, to make it worth using. Upgrading the C5 optional internal slot to be a C6 one would probably already do the trick. This small change would allow the ship to bring a C6 SLF hangar and a C6 shield at the same time.
Thanks to its 3 military compartments, it would then be a viable (and a bit cheaper) alternative to the SLF-carrying Krait MK II. It would still have a smaller fuel scoop and less versatility on what you could do with it, but from a pure combat point of view this change would put it from "a little better than the Asp Scout" to a rather good position.
And in the unlikely case that players would really react to this change, it then might also be worth having a ship kit. (I know that the "unlikely case" sounds sarcastic. But completely engineering a ship is effort, and many people have more ships already engineered for different combat purposes, without the need to do it for yet one more. )
Edit: and yes, i am aware how strange it is, that i would actually propose a buff on here. But this would be a very specific buff to a very specific ship, which for quite obvious reasons never found its place. I still at the same time support any nerfing of engineering effects. Which in the end also would help off-meta ships like the Crusader a lot.
