Short answer: package managers are highly integrated into their distribution. You may use a foreign package manager on your distribution, but this can lead to unstable, not up-to-date, redundant installation.
Long answer
A package manager keeps track of your system components and configuration:
- Librairies
- Softwares
- Configuration files
- Versions
- Architecture
This imply that a package manager knows where everything is installed and how, and respect the distribution rules when installing new packages.
Also, some package manager like Portage (on Gentoo) install packages by building them from sources. APT or RPM install pre-compiled packages.
If you use two different package managers side by side, they may override each other: install duplicate files at different places, override configuration files, replace dependencies (ex: libraries) by new versions incompatible with the rest of the system, ...
Now, in theory, you can use several packages managers on a single system. But you'll need to configure and adapt them carefully.
Other options
Tools like PackageKit provides the same interface to install and manage packages on several distribution. It doesn't replace the system's default package manager, but wrap it.
Alien can be used to convert rpm packages to deb packages.
