It was the last true Chris Reeve design and the culmination of decades of small refinements to get to that point. The new versions came after Chris left the company, so they have the brand pedigree but not the designer pedigree.
Also, the 21 had the heat treated titanium lock face, which some prefer the feel and security of (although the ceramic ball has functioned quite well in the field). The new models did away with that, possibly due to the additional cost associated with fitting the old design.
Have to laugh at some of the suggestions in this thread, some of these aren't remotely similar to a Sebenza. Even for the ones that are, nothing is exactly like the Sebenza. There are much cheaper knives that cut just as well and similarly priced knives that use the same materials, but they have advantages in different areas, not the same ones.