Because MacPorts is itself written in Tcl, so you'd have a bit of a bootstrapping problem if you had to build and install Tcl from macports before you could use macports.

Sure, it could probably be worked around by having some set of binary tcl distributions to start from, but since Tcl is well-supported by the system, why go to all the extra trouble?

Also, MacPorts isn't really planning on being cross-platform so much anymore.  There were some early experiments with this, and no one is deliberately trying to stop people from using it on, say, Linux or FreeBSD, but no work has really gone into this area either and many many ports just assume they're running on a Mac (not even "vanilla Darwin" is supported anymore).

- Jordan