Like others said, the timing and mini-game of choice over others. Zombies are well related to Halloween, and in the poll I found, the community had voted Accelerate highest over Zombie Massacre.
It would have been more than possible, but a lot of people seem to be absent lately/missing/not working as much as they used to.
That, combined with that they hope to leave EA early 2018. With 4 more planned mini-games, means they would need to release a game every 2 months now, starting with the 1st in October. You can draw an educated guess based on analysis, info gathering, and figuring out a time management plan to draw a reasonable conclusion of expected events in order to achieve said goals.