The ports tree was left out of OSX because it was never in it.
Ports is a BSD* convention. Basically, it's a series of makefiles etc. with instructions for downloading the latest source and applying OS specific patches to it, then building it. All you type is make, make isntall after you su.
The problem is that for a ports tree to exist you need people to maintain it. For BSD*, the people who make the software work make the port branch for the software and submit it to the ports tree, which is updated nightly. We would need someone to do this for Darwin.
I'm still not sure how well Apple's .pkg files are manageable, but it seems like it would be a fine way to manage software installs...