After pondering this for a while here these are what makes sense to cause the delay and silence.
- Performance was underwhelming
- Numerous AI issues
- Game Stability
- Numerous miscellaneous game play bugs
Performance was underwhelming - Given the complexities of this game being a large city with tons of AI and no loading screens, it seems likely there were issues in certain areas where the performance didn't hit the targets for consoles. Given the relative market share of consoles this was deemed unacceptable and probably a big task for their optimizations and polishing.
Numerous AI Issues - They are trying to do a LOT with AI in this game and this is also their first time tackling AI in a FPS. It makes sense in combat there are many times the AI does not behave correctly leading to a bad experience. Within the city there probably a lot of examples of AI doing 'dumb' things given the complexity. This could be quite embarrassing.
Game Stability - It seems rather likely that the game is not 100% stable and will crash during certain scenes or areas and this is unacceptable on release. Remember back during the E3 showing there were multiple mentions of the game crashing there as well. It looks like the issues were much deeper than originally thought.
Numerous miscellaneous game play bugs - The good news is the testers and QA teams are doing their jobs, it would just seem that the list of items they found to fix was much larger than anticipated and the closure of the items is going slower than anticipated.
Taking the culminations of those items they probably looked at probability of closure and ROI to releasing on April and deemed it was a bad decision. The items above would lead to negative reviews and press and hurt the sales of the game. So they probably looked at the time need to guarantee the closure on the majority of items and went with that as a replacement, also explains why so long. The game will still release with issues, it's just how it goes. It just depends on the severity of the issues.