Because OpenVZ has a philosophy of the container as a simple VM running many processes just like a server does, but Docker is being developed as a container whi

Because OpenVZ has a philosophy of the container as a simple VM running many processes just like a server does, but Docker is being developed as a container which runs one process which only communicated with other processes through defined interfaces which you configure when you create the container. Docker makes it easier to scale up and down, and to move stuff bewteen servers, but you need to do more work up front. There never is one true solution that works for everything. Typically, there are solutions which work well for all the small stuff but not so good for a few big things. Docker works great for all the small stuff. A mission critical database is often the one big thing that is the exception. Also, Docker is not the only way to handle all the small things. LXD works well. Some companies can live on AWS Lambda Functions. Looking for the Holy Grail of one ring to do it all and in the darkness bind them makes for an interesting lifelong quest, but IMHO you will never get there.