1. Activity. Halo 5 has been out for about a year now, and a number of new releases are coming out as well. The population of Halo 5 is quite low now, and this is shown in FC as well. While this is mostly just here as an excuse, it is still a correlation.
2. Motivation. Myself included, the leadership and memberbase don't have the motivation needed to maintain an active and effective war. I don't even play Halo aside from Battles or forging the map, and everyone else has been putting in very few hours compared to how they used to. Motivation is the single-most important thing for a community like us, and is needed for everything from recruitment, practice, and holding activities outside of the main battles.
3. FC is by no means dead. We're just suspending the wars. Everyone is still encouraged to use the Discord server or forums to setup gamenights, find groups to play games, etc. Ask around and you can find people playing pretty much anything.
4. Looking forward to Halo Wars 2.