Going to have to correct Guido here...
"GvR: I guess one difference is that it has a long history, since it was first distributed in 1991. In those days nobody talked about "open source", and Richard Stallman [founder of the Free Software Foundation] wasn't very well known and the GNU General Public License didn't exist."
The current version (v2) of the GPL is copyrighted June 1991, and v1 is copyrighted February 1989
(See http://www.gnu.org/licenses/gpl.html, http://www.gnu.org/copyleft/copying-1.0.html). RMS certainly was not as well know back in 1991 as he is now, but I think it is fair to say that he (and the FSF) was already by then a very significant figure in the coding world.
It is true that "open source" was not a phrase in use at that time (OSI was launched in 1998, see http://opensource.org), but that is neither here or there in relation to Free Software.