Much of the technology was there, but Docker was able to achieve a critical mass, with streamlined workflows. Perhaps as much a social phenomenon as a technical one?

Docker is more than just chroot. You also need: overlay file system; OCI registry and community behind it, to create thousands of useful images. And, of course, the whole idea of creating images layer by layer and using immutable images to spawn mutable containers.

I don't actually think that you need network or process isolation. In terms of isolation, chroot is enough for most practical needs. Network and process isolations are nice to have, but they are not essential.