Comment by kqr

The momentum, I think, matters a lot more than the other two. Makefiles already allowed you to build layered things with caching. Artifact repositories have also long been a thing.

But the argument I used for advocating Docker in environments where people were not fans of it was that the Linux container had become the de facto standard unit of deployment. There is so much tooling written under that assumption, and if you want to use any of that tooling, you need to package your deployments as Linux containers, even if you want nothing to do with Docker otherwise. That's why it had so much momentum.

Then a fourth point I think you're missing is that the concepts used by Docker are relatively simple. Before I understood what Linux containers were, I used to think things like, "Why would I care about Docker when I can isolate my processes with Linux users, cgroups, namespaces, etc." What I didn't know at that time is that "Linux users, cgroups, namespaces, etc." is exactly the thing Docker was giving a friendly UI to.