1.3 still has a pretty big userbase. Once source came out it kind of killed all that was DoD because it was a major system hog, and it was a completely different game, not just a port. So of the hundreds of clans that played and wanted to play it in source, they either couldnt or just kept playing 1.3 and gradually started falling off the face of the earth.
Hell, i played 1.3 with the same clan for 3 years, and pretty much right when source came out we broke up (didnt help that wow came out a little before then) because it was no longer dod, it was dod2, and it sucked. Still does.