I assume it was because it would take a lot of money to promote another title fight, and that was during a time when the UFC was in deep financial stress and couldn't afford to spend that kind of money on something that wasn't a guaranteed draw-- meaning a title fight not involving Matt Hughes or Randy Couture or Tito Ortiz (or, to a lesser extent, Tim Sylvia). The UFC didn't even have a MW division during that time cuz', with Baroni losing whenever he was given a chance at or near the title (sadly; it would've been awesome to have Baroni as the MW champion) and Bustamante leaving for Pride, they didn't have any profitable, marketable MWs, and didn't have the money to take a chance on finding and promoting another one for the title.
I don't really know, though, and I'm curious as well.