Why it is recommended to run only one process in a container?

In many blog posts, and general opinion, there is a saying that goes "one process per container".

Why does this rule exist?Why not run ntp, nginx, uwsgi and more processes in a single container that needs to have all processes to work?

blog posts mentioning this rule:

• "Single-­process-­per-­container is a recommended design pattern for Docker applications."
• "Docker is only for creating single-process or single-service containers."
• "better to use one process per container"
• "Run a single service as a container"
• "One process per container"
• "one process per container"