I’ve watched at the forum and didn’t happen to find the answer to this question, although I guess it has been answered before (probably many times).
I assume models like the a225 or the a124 could be fleet-excluded as they represent, by far, the best way to farm XP, but I don’t know the posible reason behind other mentioned models.
Thank you for reading! 
I have a random class 1 contract at MCT from a player who has a A124 in his fleet at LEJ. No idea how
Premium pass
My guess is that since the planes you mentioned already have a very similar and maybe more popular alternative which is puchasable (for the examples you mentioned it would be AT76, B744F, AT46, DH8B and B752F), or the planes wouldn’t really offer any major routing benefits, the devs probably just didn’t see them as high-enough-priority to make all of the player liveries for them. I’m sure that takes a good bit of time and for the negligible usage the planes would receive it might just not be worth it. This is just an educated guess though, there might be a completely different reason.
Either that or they’re just special planes, like the An-225 and Airbus Beluga. I assume when the BLCF comes out it will work in the same way - premium pass or events only.