Why:
Because the game is not structured to allow more than 8 people in a party locally. You cannot be in a party with more than 8 people. To make it so you can they would have to do one of the following:
1. Allow an alliance with up to two 8 person parties: Which would require adding alt/ctrl support for targeting people in the 2nd and 3rd groups. Significant coding effort that would push the patch back even further. Because they didn't plan on this out of the gate. Or they were working on it and the patch got pushed back and they cut this out to get it launched.
2. Redo the Duty Finder entirely so that we don't have to be in a party to use it. I favor this idea. They should make the duty finder like a match selector from League Of Legends. E.g. Let's say you want to go to Crystal tower and you want to organize your own run so you go into the Duty Finder and select Crystal Tower. Then in there you click "Create Group" instead of "Join Group" where you get a new screen. At the top it would say "Crystal Tower" and at the bottom would be 3 sections, 1 for each group of 8. You would then invite a leader to group 2 and group 3. They would join you in the duty finder window. You would then invite your 7 people from the FC and they would invite their 7 people "All in the same FC" until all 24 people are in the Raid Creation Screen.
Then you would click Start and everyone is qued into crystal tower and zones in for the raid.
I really wish they would work on #2 for Everything.
