Just based on my own experiences, I would say that they had to rewrite the entire code base, because D1's was a horrid mess. that means that working on changes in the future will be much faster and easier for the team. But it also means they had to re-code the entire base of the game, and that means picking the critical features to go in at launch, and which ones to add in a future enhancement. i would guess a lot of the things we are missing didn't make the timeline cut, in the interest of delivering something functional and playable on all the release dates.