Why not? dpkg is already been used by jailbroken devices in Cydia.

Actually it seems very likely that one or more of the popular linux distro package manager ecosystems would fare well on other OSs. Arch Linux's pacman was ported to Windows, e.g..

It's absolutely possible to install yum on OS X (I know this from experience)

Why not? A friend of mine was employed by a Very Large Company at one point to (amongst other things) maintain their AIX port of rpm/yum.

Not literally yum, but something with a similar design rather than abusing a git repo.