I think people really undervalue the base design of k8s that is responsible for the existence of CRDs and other related pluggability (even before TPR became CRDs).
Pretty much every other option was more closed and with no extensibility, plus docker swarm was plagued (at least from my PoV) with stories of instability... and I dunno about others, but I and various people I talked with were burned with running docker in production, something that k8s nicely repackaged removing a lot of the things that were problematic.
All this went into giving some serious base beyond marketing and PR - the closest I've seen from other players is classic Rancher and Nomad, and especially the latter seems much less capable.
Pretty much every other option was more closed and with no extensibility, plus docker swarm was plagued (at least from my PoV) with stories of instability... and I dunno about others, but I and various people I talked with were burned with running docker in production, something that k8s nicely repackaged removing a lot of the things that were problematic.
All this went into giving some serious base beyond marketing and PR - the closest I've seen from other players is classic Rancher and Nomad, and especially the latter seems much less capable.