CF is a complete PaaS, isn't it? So it's not directly comparable to Docker nor Kubernetes.

From my perspective, CF seems far too large, monolithic (as in all-or-nothing) and opinionated, and honestly a bit heavy-handed and antiquated (the last two partly due to BOSH).

What's attractive about K8s in particular is the elegant, layered, modular, unopinionated design. K8s itself isn't simple, but it's quite small and lightweight, and its core principles are easy to understand; more importantly, it offers primitives without imposing a particular framework on anything (for example, it doesn't care about how the underlying host is configured or provisioned), which makes it a superb foundation for a higher-level PaaS, once someone designs that (maybe Deis).