This might not make much sense but I did try comparing the gap between the first announcement and full game release date of three recent games, HM, ULiL and AoCF.
It takes 8 months but it was released unfinished. It takes extra 3 months to include Kokoro and 1 extra spell card for everyone else. I think Tasofro release a minor balance patch after another month but I'm not really sure. Anyway, HM takes around 10-11 months to complete. Make sense for it to take that long since HM starts everything from scratch.
ULiL takes 7 months to release, in a complete state to boot because it uses the same sprite as HM. However, tweaking UI, including characters' personal Urban Legend attack, new story modes, attacks, stages, and adding Mokou and Shinmyoumaru as new characters are definitely what takes up the 7 months.
Now, we come to AoCF, which somehow takes a year for it to be released despite using the same sprites/attacks from ULiL. From what I see, 5 new characters, stages and movesets added are the main things I figure would take the most time but other than that, I can't see why this game would take more development time than its predecessors and still run out of time for comiket. Here's my thoughts.
Story mode: Judging from ULiL, story mode spells will probably be the same, with 1 extra spell only for certain playable characters.
Dialogue in story and VS: If Tasofro, not ZUN, is the one typing the dialogue into the game, then this probably makes the most sense. With 19 playables, writing all that would be time-consuming. However, Soku did the same but it was finished in a month. (Of course, I'm sure that they start developing the game really early beforehand.)
Creating a really stable multiplayer network: Probably.
So, really, I have no idea why it takes a year for AoCF to develop, yet still not in time for comiket. Anyhow, I'm not a game developer so I could be missing something obvious.