I'm not an avid container user, but I think at least part of the popularity for certain dev/ops/CI use cases is how easy it is to get stuff in and out of them using bind mounts and the like. For example, having a run environment that's totally different from a build environment. The only way to do this with conventional VMs is to access them over some in-band protocol like SSH (or to have to re-build the run rootfs and reboot that machine, which is typically extremely slow).