My assumption is because Microsoft released Project 2007 at the end of
2006. While Project 2003 is still supported, I don't imagine
Microsoft would continue to have trial versions of "old" software
available.
You might see if any books about Project contain a limited trial
version of Project 2003.
I hope this helps. Let us know how you get along.
Julie
Project MVP
Visit
http://project.mvps.org/ for the FAQs and additional information
about Microsoft Project