From the wiki "Community score tallies the Honor Points collected by all players in your world. There are five Community goals for a world to strive for. Each of them carries a unique reward with a long duration. Each goal is shown by its own symbol."
This is a rather odd way to operate an event. This discourages/penalizes most players in the world from participating in the event. The largest alliance (or player) in the world stands to gain the most while the smaller alliances, by participating in this event, are only helping their enemy (the largest alliance) to gain more power and grow stronger than them. Better for smaller alliances to ignore this event, even though they would fail to get any of the personal rewards.
An example of when a "community reward" kicks in:
Large alliance = 5000 cities x 60% more resources
Smaller alliance = 2000 cities x 60% more resources
Do the math and who do you think is better off?
It's the same problem the game has when you are on an island with many enemy players and you spend resorces to increase the farm levels; you are only helping your enemy to become stronger than yourself; doesn't make sense.