Primarily because anything historically that the docker tab / plugins tab can do, CA does better without the inherent design limitations. Right now you have two completely separate areas that function differently for additional apps to your server, rather than a unified area and interface for them.
While there are significant design differences between a plugin and a container, IMO they are fundamentally the same (simply an app you're installing) and are just different delivery mechanisms.
As I said, it's a long term goal.