MOSTLY because docker swarm is not ready for scheduling.
SECOND because fleet cooperates with systemd giving a single solution for the timebeing
AND while docker and coreos work together there is some discourse. I'm trying to have a balance between rkt and docker so that if we have to select a winner that the amount of work is minimal.
ALSO Swarm has it's origins in a project called flynn. I used it for a while however it was only marginal.
FINALLY, I'm currently using fleet for the Z reporting and it seems to be working well