there are a few:
pack size
work load
game play
Pack size
The littlebird pack had 5 choppers and where a little less than 6 Meg. The current Kiowa pack have 5 choppers according to your list so how much bigger can the kiowa pack be vs the LB pack and how much more space would one more chopper use.
Work load
Well it would use some time making but wouldnt be that much i would think. You already have the weapons and the chopper models, so wouldnt it just be a Copy & paste job more or less?? And the same thing with the config since i would think they where made out aswell??
Game play
Hmmm this isnt really a reason is it?? If it is there in real life there's also room for it in the game. Also since this is your mission
To make realistic, high quality, highly accurate and highly detailed addons
i see a even bigger reason to see it in game. Also with the LB pack you didnt just make the versions with minigun/FFAR and the one with outside seats, but you also made the 2 Hellfire/FFAR/minigun and the MD500D TOW versions.
Just thought of something that could make me unpopular with some since it would delay the Kiowa pack. But here goes. You should only have one unarmed chopper as a base model. Then have it so you could add the weapon mounts you want that have the ammo included. Just like this.
Weapon mounts:
Duel Hellfire mount (with two hellfire's)
FFAR Pod (with 7 rockets)
Duel Stinger Mount (with 2 stinger's)
50cal Pod (with the ammount of shots one of these have
)
And then either some ammocrates or a support vehicle that you could place near a helipad or where you want it to land and rearm.
Reasons for this would be realism. The Real Life version have two weapon arms that can attach these four weapon mounts. It doesnt need new arms for each weapon since the attachment for these weapons are the same. So basicly you could make what version you want, and change it in a mission if needed.
I would rather see something like this and wait for it, if it can be done.