Pardon boundaries of my knowledge, but I wonder why still only one instance of any package manger can be run on a system? Why multiple installations are not possible? If there's a way to achieve this it'll be really helpful..
Mostly because it's possible for packages to make changes to shared system files, and currently there's no method for keeping track of which shared system files the packages' contents, pre-installation or removal scripts, or post-installation or removal scripts make changes to.
If two packages try to modify the same system file at the same time, there is a good chance that the file will be mangled and broken, leading to very bad behavior of the system. Updating the graphics driver and the X Server package at the same time may render the system unable to display a graphical login / interface for the user! This would be very bad.
In theory, you could keep track of this information, and run everything in parallel except for the packages that have to be run one-at-a-time, but this adds a considerable amount of complexity to the package management software and dependency resolution, and generally isn't worth it for the benefits. Usually, the bottleneck is somewhere else - downloading the packages, or the harddrive. If your harddrive is slow, installing packages in parallel can actually slow down package installation.