I thought the obvious reason was storage. I don't see it mentioned here, but storage is a huge pain point. How do you store your critical data "with Docker" is a labyrinthine set of steps.

Docker's answer to storage so far has been "don't use Docker". That's their answer. Use volumes to map some other storage, but then you have to have some way of mapping storage to containers outside of Docker. Now you're really stuck.

Containers are awesome, but unless your product doesn't do work, you'll need to store data at some point. And that's when the magic stops.