Because Publisher:
(1) is primarily a desktop publishing program developed for print documents
(2) is a Microsoft program as is Explorer

There are things you can do to make it a bit more cross-browser friendly.
You would have to search the achieves of this group for those suggestions
tho. I openly admit that I use FrontPage for websites.

Pub 2003 has limited cross browser support, and is designed to work best
with IE. Some people have better luck than others, but about your only
option is to do away with the wizard generated navbar and create your own
navbar with absolute links. There is an article about creating a textual
menu here:
http://msmvps.com/blogs/dbartosik/archive/2006/01/16/81255.aspx

DavidF