Combinations that are too powerful are removed to prevent abuse. A hulk can not have carp projectile protection, or a double esword, a bunch of powerful genetics powers will cause you to die from genetic instability, carp should not be able to be combined with antags such as changelings or vampires, vampires especially.
Vampires are very dangerous in close quarters. They have excellent stun abilities with an AOE glare, a stunning shriek. They can summon bats to help them escape, they can shadowstep out or mist form away. They have an anti stun, they have thermal vision. A single full feeding will give most of these powers. Combining this with immunity to all projectiles is ridiculous. Now the obvious argument against this is that vampires dont have easy access to hulk, they don't get telecrystals or an uplink. While that is true, currently it is not working out that way. People are purposefully seeking out traitors to get carp fist, usually in exchange for completing their objectives. As well with the current round balance, traitors+ rounds are very common so having vamps and traitors in the same round is fairly likely. An obvious solution would be to crack down on friends seeking each other out to try to code word each other to get their carp, but that doesn't appear to be against the rules technically and much more importantly would require a ton of work from the admins. The simplest solution is simply to make it so a vampire can not use a carp scroll. If they want to get syndie equipment they still can to help them, but not something quite that incredibly over powered.